Vacuum Electrospray Research Articles

Development of a high‐performance electrospray droplet beam source

The electrospray droplet impact (EDI) method was developed as a new massive cluster source, but the current EDI method lacks adequate beam focusing and density to achieve imaging and depth profiling with sufficient spatial resolution and within a reasonable time. To improve EDI beam performance, we have developed a technique for stable electrospraying of volatile liquids under vacuum. However, as is well known, vacuum electrospray of volatile liquids is extremely difficult because the evaporative cooling causes freezing of the volatile liquids in vacuum, and to prevent the freezing of these liquids, we irradiated the tip of the electrospray emitter with an infrared (10.6 µm) laser. The tip of the emitter was directly observed with optical microscopy, and the droplet ion beam current was measured with a Faraday cup. A stable vacuum electrospray of pure water was achieved with continuous laser irradiation. Also, typical modes of electrospray were clearly observed with the microscope even under vacuum conditions. The addition of 0.01 M trifluoroacetic acid to water caused an increase in current by at least one order of magnitude compared with water only. From these results, the vacuum electrospray technique can be expected to be a high‐performance massive cluster ion beam source. Copyright © 2012 John Wiley & Sons, Ltd.

Vacuum electrospray of volatile liquids assisted by infrared laser irradiation

Current large cluster sources such as C(60) or argon utilize gas-phase sources which are of low-brightness and cannot be focused efficiently to better than 1 micron diameter spot size. The development of a high-brightness large cluster ion source is of critical importance to achieve high resolution in secondary ion mass spectrometry (SIMS) imaging of organics. We propose a new high-brightness large cluster ion source, and a technique for producing a stable electrospray of volatile liquids under vacuum. It is known that vacuum electrospray of volatile liquids such as water is extremely difficult because of freezing of the liquids introduced in vacuum by evaporative cooling. To avoid freezing, the tip of the electrospray emitter was irradiated by a continuous wave infrared laser. Without continuous laser irradiation the vacuum electrospray of a water/methanol solution was unstable with respect to the shapes of the Taylor cone and current, whereas continuous laser irradiation produced a stable electrospray of water. The typical modes of electrospray were clearly observed with an optical microscope even under vacuum conditions. A stable vacuum electrospray could be achieved by improving the vacuum pressure to suppress electric discharge and by using the laser to maintain the liquid state. This is the first description of the production of a stable vacuum electrospray of volatile liquids such as water. This vacuum electrospray technique can be expected to produce a novel high-brightness large cluster ion beam source.

A single centre water splitting dye complex adsorbed on rutile TiO2(110): Photoemission, x-ray absorption, and optical spectroscopy

A single centre water splitting dye complex (aqua(2,2'-bipyridyl-4,4'-dicarboxylic acid)-(2,2':6',6''-terpyridine)Ruthenium(II)), along with a related complex ((2,2'-bipyridyl-4,4'-dicarboxylic acid)-(2,2':6',6''-terpyridine)chloride Ruthenium(II)), has been investigated using photoemission and compared to molecules with similar structures. Dye molecules were deposited in situ using ultra-high vacuum electrospray deposition, which allows for the deposition of thermally labile molecules, such as these dye molecules. Adsorption of the dye molecules on the rutile TiO(2)(110) surface has been studied using core-level and valence photoemission. Core-level photoemission spectra reveal that each complex bonds to the surface via deprotonation of its carboxylic acid groups. A consideration of the energy level alignments reveals that both complexes are capable of charge transfer from the adsorbed molecules to the conduction band of the rutile TiO(2) substrate.

Charge transfer dynamics of model charge transfer centers of a multicenter water splitting dye complex on rutile TiO2(110)

Charge transfer dynamics between an adsorbed molecule and a rutile TiO(2)(110) surface have been investigated in three organometallic dyes related to multicenter water splitting dye complexes: Ru 535 (cis-bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylato)-ruthenium(II)), Ru 455 (cis-bis(2,2'-bipyridyl)-(2,2'-bipyridyl-4,4'-dicarboxylic acid)-ruthenium(II)), and Ru 470 (tris(2,2'-bipyridyl-4,4'-dicarboxylic acid)-ruthenium(II)). The adsorption of the dye molecules on the rutile TiO(2)(110) surface has been studied using core-level and valence photoemission. Dye molecules were deposited in situ using ultrahigh vacuum electrospray deposition. Core-level photoemission spectra reveal that each complex bonds to the surface via deprotonation of two carboxylic groups. All three dye complexes show evidence of ultrafast charge transfer to the TiO(2) substrate using the core-hole clock implementation of resonant photoemission spectroscopy.

Single molecule magnets on a gold surface: in situ electrospray deposition, x-ray absorptionand photoemission

Single molecule magnets based on the dodecamanganese (III, IV) cluster withtwo different types of ligand (acetate and benzoate) have been studied on theAu(111) surface. Due to the non-volatile and fragile nature of the molecules,we have used ultra-high vacuum electrospray deposition to produce a series ofsurface coverages from a fraction of a monolayer to multilayer films in both cases.Synchrotron radiation based electron spectroscopy has been used to study theadsorption of the molecules on the Au(111) surface and the effect that this has on theoxidation states of the manganese atoms in the core. In both cases, reduction ofthe core is observed due to the interaction with the surface, to essentially thesame extent despite substantial differences in the size and nature of the ligandshell.

Characteristics of a charged-droplet beam generated by vacuum electrospray of an ionic liquid

To develop a cluster ion beam source for secondary ion mass spectrometry (SIMS), beam characteristics of charged droplets electrosprayed in a vacuum were investigated using a room-temperature molten salt (i.e., an ionic liquid). A target current exceeding 200nA was produced at a capillary voltage of 1.5kV even with a low flow rate of 0.05μL/min. Efficiency of beam generation varied with increasing capillary voltage and flow rate. Additionally, m/z values of emitted charged particles were investigated by a time-of-flight technique. In addition to masses and charge numbers, radii of the charged droplets were examined.

X-ray absorption and photoemission spectroscopy of zinc protoporphyrin adsorbed on rutile TiO2(110) prepared by in situ electrospray deposition

Zinc-protoporphyrin, adsorbed on the rutile TiO(2)(110) surface, has been studied using photoemission spectroscopy and near-edge absorption fine structure spectroscopy to deduce the nature of the molecule-surface bonding and the chemical environment of the central metal atom. To overcome the difficulties associated with sublimation of the porphyrin molecules, samples were prepared in situ using ultrahigh vacuum electrospray deposition, a technique which facilitates the deposition of nonvolatile and fragile molecules. Monolayers of Zn protoporphyrin are found to bond to the surface via the oxygen atoms of the deprotonated carboxyl groups. The molecules initially lie largely parallel to the surface, reorienting to an upright geometry as the coverage is increased up to a monolayer. For those molecules directly chemisorbed to the surface, the interaction is sufficiently strong to pull the central metal atom out of the molecule.

A new cluster-ion-beam source for secondary ion mass spectrometry (SIMS) using the electrospray of a pure ionic liquid under high vacuum

Electrospray characteristics were studied using a pure room-temperature molten salt (i.e., an ionic liquid) at pressures around 10 −5 Pa as well as at atmospheric pressure. The ionic liquid N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide was tested, which has negligible vapor pressure and high conductivity. During electrospray, the ionic liquid was introduced at a constant flow rate into a stainless-steel capillary (i.d. 30 μm). It was demonstrated that stable electrosprayed currents exceeding ±1 μΑ were continuously produced in both positive and negative modes. The electrosprayed currents in a high vacuum were twice those at atmospheric pressure. It was found that gas pressure rose slightly with increasing electrosprayed currents. Residual gas analysis revealed that gas component at negative mode was different from that at positive mode. Experimental results indicate that vacuum electrospray of pure ionic liquids is applicable to a massive-cluster beam source for SIMS.

Adsorption of a Ru(II) dye complex on the Au(111) surface: Photoemission and scanning tunneling microscopy

The adsorption of the dye molecule N3 [cis-bis(isothiocyanato)bis(2,2(')-bipyridyl-4,4(')-dicarboxylato)-ruthenium(II)] on the Au(111) surface has been studied using core-level and valence photoemission and scanning tunneling microscopy (STM). The dye molecules were deposited in situ using ultrahigh vacuum electrospray deposition. The core-level spectra reveal that the molecule bonds to the surface via sulfur atoms with no deprotonation of the carboxylic groups. The STM images show that at low coverage the molecules decorate the Au(111) herringbone reconstruction and form uniform monolayers as the coverage is increased.

Nanojets, Electrospray, and Ion Field Evaporation: Molecular Dynamics Simulations and Laboratory Experiments

The energetics, interfacial properties, instabilities, and fragmentation patterns of electrosprays made from formamide salt solutions are investigated in a mass spectrometric vacuum electrospray experiment and using molecular dynamics (MD) simulations. The electrospray source is operated in a Taylor cone-jet mode, with the nanojet that forms being characterized by high surface-normal electric field strengths in the vicinity of 1 V/nm. Mass-to-charge ratios were determined for both positive and negative currents sprayed from NaI-formamide solutions with solute-solvent mole ratios of 1:8.4 and 1:36.9, and from KI-formamide solutions with mole ratios of 1:41 and 1:83. The molecular dynamics simulations were conducted on isolated 10 nm NaI-formamide droplets at mole ratios of 1:8 and 1:16. The droplet was subjected to a uniform electric field with strengths ranging between 0.5 and 1.5 V/nm. Both the experiments and simulations demonstrate a mixed charge emission regime where field-induced desorption of solvated ions and charged droplets occurs. The macroscopic parameters, such as average mass-to-charge ratio and maximum surface-normal field strengths deduced from the simulations are found to be in good agreement with the experimental work and consistent with electrohydrodynamic theory of cone-jets. The observed mass spectrometric Na (+) and I (-) solvated ion distributions are consistent with a thermal evaporation process, and are correctly reproduced by the simulation after incorporation of the different flight times and unimolecular ion dissociation rates in the analysis. Alignment of formamide dipoles and field-induced reorganization of the positive and negative ionic charges in the interfacial region are both found to contribute to the surface-normal field near the points of charge emission. In the simulations the majority of cluster ions are found to be emitted from the tip of the jet rather than from the neck region next to the Taylor cone. This finding is consistent with the experimental energy distributions of the solvated ions which demonstrate that indeed most ions are emitted closer to the jet region, that is, beyond the cone-neck region where ohmic losses occur. This observation is also consistent with continuum electrohydrodynamic predictions of cluster-ion evaporation at surface regions of high curvature and therefore maximum surface electric field strengths, which may be the cone-neck region, the breakup region of the jet (usually near the tip of the jet), or the emitted charged droplets. In the nanoscale jets observed in this study, the regions of highest spatial curvature are at the ends of the jets where nascent drops either are forming or have just detached.

Photoemission, resonant photoemission, and x-ray absorption of a Ru(II) complex adsorbed on rutile TiO2(110) prepared by in situ electrospray deposition

An experimental study of the bonding geometry and electronic coupling of cis-bis(isothiocyanato)bis(2,2(')-bipyridyl-4,4(')-dicarboxylato)-ruthenium(II) (N3) adsorbed on rutile TiO(2)(110) is presented, along with supporting theoretical calculations of the bonding geometry. Samples were prepared in situ using ultrahigh vacuum electrospray deposition. Core-level photoemission spectroscopy was used to characterize the system and to deduce the nature of the molecule-surface bonding. Valence band photoemission and N 1s x-ray absorption spectra were aligned in a common binding energy scale to enable a quantitative analysis of the bandgap region. A consideration of the energetics in relation to optical absorption is used to identify the photoexcitation channel between the highest occupied and lowest unoccupied molecular orbitals in this system, and also to quantify the relative binding energies of core and valence excitons. The core-hole clock implementation of resonant photoemission spectroscopy is used to reveal that electron delocalization from N3 occurs within 16 fs.

Electrospray Deposition of C60 on a Hydrogen-Bonded Supramolecular Network

An electrospray technique has been successfully used to deposit C60 onto a hydrogen-bonded supramolecular template held in an ultrahigh vacuum environment. Characterization of the surface by in situ scanning tunneling microscopy shows the size-selective trapping of C60 dimers and heptamers within a 3,4,9,10-perylene tetracarboxylic diimide (PTCDI)/melamine network. We demonstrate that ultrahigh vacuum electrospray deposition (UHV-ESD), where energetic molecules impact onto a substrate, is compatible with surfaces which have been functionalized through the incorporation of hydrogen-bonded templates.

Vacuum electrospray ionization study of the ionic liquid, [Emim][Im

An analysis is presented of positive and negative vacuum electrospray currents from the ionic liquid, [Emim][Im], wetted on a sharp tungsten needle. The source is operated in a cone–jet configuration established with a low extraction voltage of approximately ±1 kV. Emission current and mass flow are measured as a function of the emitter angle with respect to the instrument axis. Mass spectra of field evaporated ions and mass-resolved energy distributions of emitted charges are recorded as a function of emitter angle. The measurements at both source polarities indicate that the cone–jet emits a mixture of ions and charged droplets. While the charged droplets are emitted with a narrow angular distribution centered on axis, the angular distributions of ions are significantly broader than those of the droplets and centered off axis. Pure ion emission is observed at emission angles larger than ∼15°. The main mass spectral peaks are from X ±{[Emim][Im]} n ions (X = Emim or Im for positive and negative polarities, respectively) with n = 0, 1, and 2. Quantum chemical calculations of the n = 0 and 1 ions and the isolated ion pairs are presented, yielding thermochemical information on the observed ionic species. The present results demonstrate that the cone–jet emits droplets from the tip of the jet, and provide direct evidence that ions are produced in an Iribarne–Thomson field-evaporation mechanism at the transition region between the Taylor cone and jet. Electrochemical modification of the cone–jet emitter properties are observed when maintaining the emitter at a positive polarity for a prolonged (minutes to hours) period. New ions emerge consisting of clusters containing a neutralized Im −. The associated current suppression can be mitigated through polarity alternation at 1 Hz. Measurements are also presented with the tungsten needle heated to 373 K, thereby reducing the liquid viscosity and increasing the flow rate. The average mass of the droplets is observed to increase with temperature while the mass distributions of the ions are not found to change noticeably.

Electrospray deposition in vacuum

We have used the established technique of electrospray in developing a portable vacuum electrospray system which can deposit, in vacuo, dissolved molecules onto a sample which may then be analysed by UHV techniques. As an initial test of the system we have analysed silicon samples with an electrosprayed layer of poly(ethylene) oxide (PEO) using atomic force microscopy (AFM). The polymer forms different structures depending on the voltage applied to the emitter, and solution composition. The system is part of our ongoing effort to deposit other materials such as nanoparticles, and large dye molecules for developing molecular dye sensitised solar cells.