Electron-Beam-Induced Damage in Self-Assembled Monolayers

Patterning Quantum Dots via Photolithography: A Review

Pixelating patterns of red, green, and blue quantum dots (QDs) is a critical challenge for realizing high-end displays with bright and vivid images for virtual, augmented, and mixed reality. Since QDs must be processed from a solution, their patterning process is completely different from the conventional techniques used in the organic light-emitting diode and liquid crystal display industries. Although innovative QD patterning technologies are being developed, photopatterning based on the light-induced chemical conversion of QD films is considered one of the most promising methods for forming micrometer-scale QD patterns that satisfy the precision and fidelity required for commercialization. Moreover, the practical impact will be significant as it directly exploits mature photolithography technologies and facilities that are widely available in the semiconductor industry. This article reviews recent progress in the effort to form QD patterns via photolithography. The review begins with a general description of the photolithography process. Subsequently, different types of photolithographical methods applicable to QD patterning are introduced, followed by recent achievements using these methods in forming high-resolution QD patterns. The paper also discusses prospects for future research directions.

Additive Lithography-Organic Monolayer Patterning Coupled with an Area-Selective Deposition

The combination of area-selective deposition (ASD) with a patternable organic monolayer provides a versatile additive lithography platform, enabling the generation of a variety of nanoscale feature geometries. Stearate hydroxamic acid self-assembled monolayers (SAMs) were patterned with extreme ultraviolet (λ = 13.5 nm) or electron beam irradiation and developed with ASD to achieve line space patterns as small as 50 nm. Density functional theory was employed to aid in the synthesis of hydroxamic acid derivatives with optimized packing density to enhance the imaging contrast and improve dose sensitivity. Near-edge X-ray absorption fine structure spectroscopy and infrared spectroscopy reveal that the imaging mechanism is based on improved deposition inhibition provided by the cross-linking of the SAM to produce a more effective barrier during a subsequent deposition step. With patterned substrates composed of coplanar copper lines and silicon spacers, hydroxamic acids selectively formed monolayers on the metal portions and could undergo a pattern-wise exposure followed by ASD in the first combination of a patternable monolayer with ASD. This material system presents an additional capability compared to traditional ASD approaches that generally reflect a starting patterned surface. Furthermore, this bottoms-up additive approach to lithography may be a viable alternative to subtractive nanoscale feature generation.

Nanoscale Patterning of Colloidal Nanocrystal Films for Nanophotonic Applications Using Direct Write Electron Beam Lithography

The small size of colloidal nanocrystal quantum dots (QDs) leads to a variety of unique optical properties that are well-suited to nanophotonics, including bright, tunable photoluminescence (PL). However, exploring the properties of solid QD assemblies at the nanoscale has proven challenging because of the limitations in the nanoscale QD patterning methods. Generally, the precise placement of QD solids is difficult to achieve, especially for tall structures with multiple QD layers, and when it is achieved the patterns often cannot withstand the further processing steps required for final device construction. Direct electron beam lithography of QDs has emerged as a straightforward patterning process that does not require ligand exchange and results in structures that retain bright PL. Here, we demonstrate that direct patterning QD films on substrates treated with a self-assembled monolayer of octadecyltrichlorosilane allows us to create feature sizes as thin as 30 nm with heights of multiple layers and characterize the pattern resolution, robustness, and placement accuracy. These structures withstand sonication in a variety of solvents, and the structures are placed within 20 nm of their intended location nearly 100% of the time. We further show how this patterning method can be applied to nanophotonics by measuring the complex refractive index of the QD materials to model the absorption and scattering cross sections of QD structures of various sizes and shapes. These simulations reveal that edge effects arising from the finite shape of the QD nanostructure lead to substantial absorption enhancement when compared to an equivalent volume region taken from a continuous QD film. Finally, we explore more complex structures by patterning QD arrays, multilayer QD structures, and QD disks inside plasmonic resonators.

Plasmon-Driven Photocatalysis Leads to Products Known from E-beam and X-ray-Induced Surface Chemistry

Plasmonic metal nanostructures can concentrate incident optical fields in nanometer-sized volumes, called hot spots. This leads to enhanced optical responses of molecules in such a hot spot but also to chemical transformations, driven by plasmon-induced hot carriers. Here, we employ tip-enhanced Raman spectroscopy (TERS) to study the mechanism of these reactions in situ at the level of a single hot spot. Direct spectroscopic measurements reveal the energy distribution of hot electrons, as well as the temperature changes due to plasmonic heating. Therefore, charge-driven reactions can be distinguished from thermal reaction pathways. The products of the hot-carrier-driven reactions are strikingly similar to the ones known from X-ray or e-beam-induced surface chemistry despite the >100-fold energy difference between visible and X-ray photons. Understanding the analogies between those two scenarios implies new strategies for rational design of plasmonic photocatalytic reactions and for the elimination of photoinduced damage in plasmon-enhanced spectroscopy.

Direct Mapping of Heterogeneous Surface Coverage in DNA-Functionalized Gold Surfaces with Correlated Electron and Fluorescence Microscopy

The characterization of biofunctionalized surfaces such as alkanethiol self-assembled monolayers (SAMs) on gold modified with DNA or other biomolecules is a challenging analytical problem, and access to a routine method is desirable. Despite substantial investigation from structural and mechanistic perspectives, robust and high-throughput metrology tools for SAMs remain elusive but essential for the continued development of these devices. We demonstrate that scanning electron microscopy (SEM) can provide image contrast of the molecular interface during SAM functionalization. The high-speed, large magnification range, and ease of use make this widely available technique a powerful platform for measuring the structure and composition of SAM surfaces. This increased throughput allows for a better understanding of the nonideal spatial heterogeneity characteristic of SAMs utilized in real-world conditions. SEM image contrast is characterized through the use of fluorescently labeled DNA, which enables correlative SEM and fluorescence microscopy. This allows identification of the DNA-modified regions at resolutions that approach the size of the biomolecule. The effect of electron beam irradiation dose is explored, which leads to straightforward lithographic patterning of DNA SAMs with nanometer resolution and with control over the surface coverage of specifically adsorbed DNA.

Writing on Nanocrystals: Patterning Colloidal Inorganic Nanocrystal Films through Irradiation-Induced Chemical Transformations of Surface Ligands

In the past couple of decades, colloidal inorganic nanocrystals (NCs) and, more specifically, semiconductor quantum dots (QDs) have emerged as crucial materials for the development of nanoscience and nanotechnology, with applications in very diverse areas such as optoelectronics and biotechnology. Films made of inorganic NCs deposited on a substrate can be patterned by e-beam lithography, altering the structure of their capping ligands and thus allowing exposed areas to remain on the substrate while non-exposed areas are redispersed in a solvent, as in a standard lift-off process. This methodology can be described as a "direct" lithography process, since the exposure is performed directly on the material of interest, in contrast with conventional lithography which uses a polymeric resist as a mask for subsequent material deposition (or etching). A few reports from the late 1990s and early 2000s used such direct lithography to fabricate electrical wires from metallic NCs. However, the poor conductivity obtained through this process hindered the widespread use of the technique. In the early 2010s, the same method was used to define fluorescent patterns on QD films, allowing for further applications in biosensing. For the past 2-3 years, direct lithography on NC films with e-beams and X-rays has gone through an important development as it has been demonstrated that it can tune further transformations on the NCs, leading to more complex patternings and opening a whole new set of possible applications. This Perspective summarizes the findings of the past 20 years on direct lithography on NC films with a focus on the latest developments on QDs from 2014 and provides different potential future outcomes of this promising technique.

Efficient electron-induced removal of oxalate ions and formation of copper nanoparticles from copper(II) oxalate precursor layers

Copper(II) oxalate grown on carboxy-terminated self-assembled monolayers (SAM) using a step-by-step approach was used as precursor for the electron-induced synthesis of surface-supported copper nanoparticles. The precursor material was deposited by dipping the surfaces alternately in ethanolic solutions of copper(II) acetate and oxalic acid with intermediate thorough rinsing steps. The deposition of copper(II) oxalate and the efficient electron-induced removal of the oxalate ions was monitored by reflection absorption infrared spectroscopy (RAIRS). Helium ion microscopy (HIM) reveals the formation of spherical nanoparticles with well-defined size and X-ray photoelectron spectroscopy (XPS) confirms their metallic nature. Continued irradiation after depletion of oxalate does not lead to further particle growth giving evidence that nanoparticle formation is primarily controlled by the available amount of precursor.

Nanopatterned polymer brushes by reactive writing

Polymer brush patterns were prepared by a combination of electron beam induced damage in self-assembled monolayers (SAMs), creating a stable carbonaceous deposit, and consecutive self-initiated photografting and photopolymerization (SIPGP). This newly applied technique, reactive writing (RW), is investigated with 1H,1H,2H,2H-perfluorooctyltriethoxysilane SAM (PF-SAM) on silicon oxide, which, when modified by RW, can be selectively functionalized by SIPGP. With the monomer N,N-dimethylaminoethyl methacrylate (DMAEMA), we demonstrate the straightforward formation of polymer brush gradients and single polymer lines of sub-100 nm lateral dimensions, with high contrast to the PF-SAM background. The lithography parameters acceleration voltage, irradiation dose, beam current and dwell time were systematically varied to identify the optimal conditions for the maximum conversion of the SAM into a carbonaceous deposit. The results of this approach were compared to patterns prepared by carbon templating (CT) under analogous conditions, revealing a dwell time dependency, which differs from earlier reports. This new technique expands the range of CT by giving the opportunity to not only vary the chemistry of the created polymer patterns with monomer choice but also vary the chemistry of the surrounding substrate.

Magnetic Nanoparticles: Material Engineering and Emerging Applications in Lithography and Biomedicine

We present an interdisciplinary overview of material engineering and emerging applications of iron oxide nanoparticles. We discuss material engineering of nanoparticles in the broadest sense, emphasizing size and shape control, large-area self-assembly, composite/hybrid structures, and surface engineering. This is followed by a discussion of several non-traditional, emerging applications of iron oxide nanoparticles, including nanoparticle lithography, magnetic particle imaging, magnetic guided drug delivery, and positive contrast agents for magnetic resonance imaging. We conclude with a succinct discussion of the pharmacokinetics pathways of iron oxide nanoparticles in the human body -- an important and required practical consideration for any in vivo biomedical application, followed by a brief outlook of the field.

Resistive switching of alkanethiolated nanoparticle monolayers patterned by electron-beam exposure

Nanoscale structures are fabricated by the direct electron-beam exposure of 10 nm gold nanoparticle monolayers and development in an emulsion. We observe resistive switching in these structures of up to five orders of magnitude.

Electron induced surface reactions of organometallic metal(hfac)₂ precursors and deposit purification

The elementary processes associated with electron beam-induced deposition (EBID) and post-deposition treatment of structures created from three metal(II)(hfac)2 organometallic precursors (metal = Pt, Pd, Cu; hfac = CF3C(O)CHC(O)CF3) have been studied using surface analytical techniques. Electron induced reactions of adsorbed metal(II)(hfac)2 molecules proceeds in two stages. For comparatively low electron doses (doses <1 × 10(17) e(-)/cm(2)) decomposition of the parent molecules leads to loss of carbon and oxygen, principally through the formation of carbon monoxide. Fluorine and hydrogen atoms are also lost by electron stimulated C-F and C-H bond cleavage, respectively. Collectively, these processes are responsible for the loss of a significant fraction (≥ 50%) of the oxygen and fluorine atoms, although most (>80%) of the carbon atoms remain. As a result of these various transformations the reduced metal atoms become encased in an organic matrix that is stabilized toward further electron stimulated carbon or oxygen loss, although fluorine and hydrogen can still desorb in the second stage of the reaction under the influence of sustained electron irradiation as a result of C-F and C-H bond cleavage, respectively. This reaction sequence explains why EBID structures created from metal(II)(hfac)2 precursors in electron microscopes contain reduced metal atoms embedded within an oxygen-containing carbonaceous matrix. Except for the formation of copper fluoride from Cu(II)(hfac)2, because of secondary reactions between partially reduced copper atoms and fluoride ions, the chemical composition of EBID films and behavior of metal(II)(hfac)2 precursors was independent of the transition metal's chemical identity. Annealing studies of EBID structures created from Pt(II)(hfac)2 suggest that the metallic character of deposited Pt atoms could be increased by using post deposition annealing or elevated substrate temperatures (>25 °C) during deposition. By exposing EBID structures created from Cu(II)(hfac)2 to atomic oxygen followed by atomic hydrogen, organic contaminants could be abated without annealing.

Selective terminal function modification of SAMs driven by low-energy electrons (0-15 eV)

Low-energy electron induced degradation of a model self-assembled monolayer (SAM) of acid terminated alkanethiol was studied under ultra-high vacuum (UHV) conditions at room and low (~40 K) temperatures. Low-energy electron induced chemical modifications of 11-mercaptoundecanoic acid (MUA, HS-(CH2)10-COOH) SAMs deposited on gold were probed in situ as a function of the irradiation energy (<11 eV) by combining two complementary techniques: High Resolution Electron Energy Loss Spectroscopy (HREELS), a surface sensitive vibrational spectroscopy technique, and Electron Stimulated Desorption (ESD) analysis of neutral fragments. The SAM's terminal functions were observed to be selectively damaged at around 1 eV by a resonant electron attachment mechanism, observed to decay by CO, CO2 and H2O formation and desorption. CO2 and H2O were also directly identified at low temperature by vibrational analysis of the irradiated SAMs. At higher irradiation energy, both terminal functions and spacer alkyl chains are damaged upon electron irradiation, by resonant and non-resonant processes.

Ten-nanometer dense hole arrays generated by nanoparticle lithography

Large area dense hole arrays with a feature size of ~10 nm were generated using self-assembled monolayers of nanoparticles as etch masks. To fabricate the hole arrays, monolayers of nanoparticles were irradiated by electron beam to turn surfactants into amorphous carbon, treated by acid to remove the nanoparticle cores, and then etched by CF(4) to deepen the holes. Evaporated gold films preferentially diffuse into the holes to generate gold nanoparticle arrays. However no obvious diffusion into holes was observed for a sputtered iron platinum film.

Patterning of Protein/Quantum Dot Hybrid Bionanostructures

Here we demonstrate patterning of protein/quantum dot hybrid bionanostructures via electrostatic assembly of engineered negatively charged fluorescent protein with positively charged CdSe/ZnS QD patterns formed through e-beam lithography and post-patterning modification with cationic ligands.

Metallic adhesion layer induced plasmon damping and molecular linker as a nondamping alternative

Drastic chemical interface plasmon damping is induced by the ultrathin (∼2 nm) titanium (Ti) adhesion layer; alternatively, molecular adhesion is implemented for lithographic fabrication of plasmonic nanostructures without significant distortion of the plasmonic characteristics. As determined from the homogeneous linewidth of the resonance scattering spectrum of individual gold nanorods, an ultrathin Ti layer reduces the plasmon dephasing time significantly, and it reduces the plasmon scattering amplitude drastically. The increased damping rate and decreased plasmon amplitude are due to the dissipative dielectric function of Ti and the chemical interface plasmon damping where the conduction electrons are transferred across the metal-metal interface. In addition, a pronounced red shift due to the Ti adhesion layer, more than predicted using electromagnetic simulation, suggests the prevalence of interfacial reactions. By extending the experiment to conductively coupled ring-rod nanostructures, it is shown that a sharp Fano-like resonance feature is smeared out due to the Ti layer. Alternatively, vapor deposition of (3-mercaptopropyl)trimethoxysilane on gently cleaned and activated lithographic patterns functionalizes the glass surface sufficiently to link the gold nanostructures to the surface by sulfur-gold chemical bonds without observable plasmon damping effects.

Characterization of dense arrays of chemiresistor vapor sensors with submicrometer features and patterned nanoparticle interface layers

The performance of arrays of small, densely integrated chemiresistor (CR) vapor sensors with electron-beam patterned interface layers of thiolate-monolayer-protected gold nanoparticles (MPNs) is explored. Each CR in the array consists of a 100-μm(2) interdigital electrode separated from adjacent devices by 4 μm. Initial studies involved four separate arrays, each containing four CRs coated with one of four different MPNs, which were calibrated with five vapors before and after MPN-film patterning. MPNs derived from n-octanethiol (C8), 4-(phenylethynyl)-benzenethiol (DPA), 6-phenoxyhexane-1-thiol (OPH), and methyl-6-mercaptohexanoate (HME) were tested. Parallel calibrations of MPN-coated thickness-shear-mode resonators (TSMR) were used to derive partition coefficients of unpatterned films and to assess transducer-dependent factors affecting responses. A 600-μm(2) 4-CR array with four different patterned MPN interface layers, in which the MPN derived from 7-hydroxy-7,7-bis(trifluoro-methyl)heptane-1-thiol (HFA) was substituted for HME, was then characterized. This is the smallest multi-MPN array yet reported. Reductions in the diversity of the collective response patterns are observed with the patterned films, but projected vapor discrimination rates remain high. The use of such arrays as ultralow-dead-volume detectors in microscale gas chromatographic analyzers is discussed.

Densely integrated array of chemiresistor vapor sensors with electron-beam patterned monolayer-protected gold nanoparticle interface films

Use of electron-beam induced crosslinking to pattern films of monolayer-protected gold nanoparticles (MPNs) onto a chemiresistor (CR) sensor array is described. Each of the four CRs comprises a 100 µm(2) set of interdigital electrodes (IDEs) with 100 nm widths and spaces, separated from adjacent devices by 4 µm. Films of four MPNs, each with a different thiolate monolayer, were successively patterned on the IDEs. Vapor exposures yield rapid, reversible changes in CR resistances and differential vapor sensitivities comparable to those reported for larger CRs with unpatterned MPN films. The array response patterns facilitate vapor discrimination. This is the smallest MPN-coated CR array yet reported. The advantages of using such an array as the detector in microfabricated gas chromatographic analyzers are considered.

Photoimaging with singlet oxygen at the solid-air interface

Films of anthracene carboxylic acids were irradiated through photomasks and oxidized at the exposed regions by singlet oxygen upon sensitization. The efficiency of a photomask to protect the material underneath was investigated by optical and infrared spectroscopy. As the thickness of the film is reduced, the efficiency of the mask drops. This is explained by the migration of singlet oxygen at the solid-air interface, which in turn reacts at the masked area. For films with a thickness of <15 nm, the efficiency of the mask approaches zero; sufficient efficiency is achieved at thicknesses > 100 nm. From the investigations, it will become clear that the contrast between the irradiated and masked area of an image is affected by reduction of the film thickness. On the other hand, the resolution of an image, which relates to the minimum feature size of an image, is not dependent on the thickness of the film. The contributions of "inside" and "outside" reactions are examined separately, and a quantitative approximation of the spatial range of both modes of the oxygenation is given. We set up an approximate relation between mask efficiency and experimental conditions comprising internal and external oxygen diffusion, film thickness, and mask dimensions. These results give a deeper insight into the limits of resolution and contrast in singlet oxygen lithography.

Electron resist behavior of Pd hexadecanethiolate examined using X-ray photoelectron spectroscopy with nanometric lateral resolution

Electron resist behavior of Pd hexadecanethiolate is studied by varying the e-dosage from 2-280 muC.cm(-2). The e-beam exposed resist is characterized using energy dispersive spectroscopy, infrared spectroscopy, and X-ray photoelectron spectroscopy with nanometric lateral resolution. Electron beam exposure causes defects in the alkyl chain of the thiolate, giving the required solubility contrast during the developing step, thus qualifying the precursor as an e-beam resist. On exposure to the e-beam, the reduction of Pd(2+) to Pd(0) is observed, and the reduction increases with increasing e-dosage. The resist is highly sensitive, with the estimated sensitivity being 32 muC.cm(-2). Thermolysis at 250 degrees C leads to the formation of Pd nanoparticles, demonstrating the essential feature of a direct write resist for conducting patterns.

Localization vs conduction: anionic excitations in alkanethiol self-assembled monolayers

Low-energy (6-11 eV) electron injection into Xe-coated self-assembled alkanethiol monolayers (SAMs) is reported. At most energies, the presence of the Xe film has negligible effect on the incident electrons, which penetrate the overlayer and induce significant C--H bond rupture at the terminal methyl sites and the subsurface methylene sites of the organic substrates. However, irradiation at 7.7 +/- 0.2 eV can lead to resonant electronic excitations of the Xe adsorbates to create transient anionic states in the Xe overlayer. Transfer of anionic excitations from the Xe overlayer to the SAM initially prepares excited anionic states at the terminal CH3 groups and leads to highly selective dissociations at the methyl sites, with negligible conduction along the alkane chain which would lead to subsurface C-H bond rupture at the methylene sites. These results demonstrate that the mobility of electronically excited charged states along the alkanethiol chains is significantly less than that of simple excess electrons and that highly site-selective chemical modifications can be induced by low-energy electrons in these highly homogeneous organic films.

A SERS-active nanocrystalline pd substrate and its nanopatterning leading to biochip fabrication

Nanocrystalline metallic Pd films have been produced by thermolysis of Pd hexadecanethiolate deposited on a Si surface. The particle size was in the range of 50-60 nm based on electron and atomic force microscopy. The Pd surface produced was found to be surface-enhanced Raman scattering (SERS) active with an enhancement factor of approximately 10(5). The electron-resist behavior of Pd thiolate is exploited in conceptualizing a prototype chip with patterned Pd regions that are SERS active, called "SERS bits", which host different biomolecules.

Highly conducting patterned Pd nanowires by direct-write electron beam lithography

Palladium hexadecylthiolate is shown to serve as a negative-tone direct-write electron resist to produce nanopatterns down to 30 nm. The written patterns do not deviate much from the precursor in composition, while a post-treatment at 230 degrees C in air produced metallic Pd nanowires with residual carbon less than 10% and resistivity close to the bulk value, a desirable property of interconnects in nanocircuitry. The as-written patterns contain small nanocrystals (<5 nm) in a hydrocarbon matrix, which upon annealing aggregate to form well-connected networks of larger nanocrystals (5-15 nm), thus giving rise to metallic conductivity.

Scanning electron microscopy of nanoscale chemical patterns

A series of nanoscale chemical patterning methods based on soft and hybrid nanolithographies have been characterized using scanning electron microscopy with corroborating evidence from scanning tunneling microscopy and lateral force microscopy. We demonstrate and discuss the unique advantages of the scanning electron microscope as an analytical tool to image chemical patterns of molecules highly diluted within a host self-assembled monolayer and to distinguish regions of differential mass coverage in patterned self-assembled monolayers. We show that the relative contrast of self-assembled monolayer patterns in scanning electron micrographs depends on the operating primary electron beam voltage, monolayer composition, and monolayer order, suggesting that secondary electron emission and scattering can be used to elucidate chemical patterns.

Large damage threshold and small electron escape depth in X-ray absorption spectroscopy of a conjugated polymer thin film

The information depth of near-edge X-ray absorption fine structure spectroscopy in the total electron yield mode (TEY-NEXAFS) is given by the escape depth of the TEY electrons z(TEY). This is determined by the effective ranges both of the inelastically scattered secondary electrons and of the primary excited electron before they thermalize below the vacuum level. For regioregular poly(3-hexylthiophene) (rreg-P3HT) thin films, we have measured the total electron emission efficiency to be 0.028 +/- 0.005 e/ph at an incident photon energy of 320 eV. The range of the primary electron was computed using optical dielectric-loss theory to be 7.5 nm. The range of the secondary electrons was then found by modeling to be 3.0 nm. This gives z(TEY) to be 2.5 nm, which is considerably less than the often-assumed value of 10 nm in the literature. It is also considerably smaller than the computed electron-electron scattering inelastic mean free path in the material, which suggests the predominance of electron-phonon scattering. Thus, TEY-NEXAFS has sufficient surface sensitivity to probe the frontier molecular layers of these organic conjugated polymers. In a second aspect of this report, the rreg-P3HT films have been characterized by in-situ core and valence photoemission spectroscopies and by ex-situ microattenuated total-reflection vibrational spectroscopy as a function of irradiation dose. No damage was observed in composition, bonding, orientation, and surface morphology under typical TEY-NEXAFS spectral acquisition conditions. For an integrated TEY that exceeds 2 x 10(-3) C cm(-2), however, the material degrades via alkyl side-chain dehydrogenation to unsaturated units, cross linking, ring opening of the backbone, and sulfur extrusion. Given that secondary electrons are the dominant cause of radiation damage, this exposure threshold measured by integrated TEY should also be valid at other X-ray energies.

Modification of Monomolecular Self-Assembled Films by Nitrogen−Oxygen Plasma

The modification of octadecanethiolate self-assembled monolayers on Au and Ag by nitrogen-oxygen downstream microwave plasma with variable oxygen content (up to 1%) has been studied by synchrotron-based high-resolution X-ray photoelectron spectroscopy. The primary processes were dehydrogenation, desorption of hydrocarbon and sulfur-containing species, and the oxidation of the alkyl matrix and headgroup-substrate interface. The exact character and the rates of the plasma-induced changes were found to be dependent on the substrate and plasma composition, with the processes in the aliphatic matrix and headgroup-substrate interface being mostly decoupled. In particular, the rates of all major plasma-induced processes were found to be directly proportional to the oxygen content in the plasma, which can be, thus, considered as a measure of the plasma reactivity. Along with the character of the observed changes, exhibiting a clear dominance of the oxidative processes, this suggests that the major effect of the oxygen-nitrogen downstream microwave plasma is provided by reactive oxygen-derived species in the downstream region, viz. long-living oxygen radicals and metastable species.

Ambient-pressure vapor deposition of octanethiol self-assembled monolayers

Scanning tunneling microscopy was used to characterize self-assembled monolayers (SAMs) of octanethiol on Au(111), created using vapor deposition at elevated temperatures and ambient pressure. Monolayers contained large, close-packed ( radical3 x radical3)R30 degrees domains with sizes considerably larger than those typically formed from conventional solution-phase preparation and with crystallographically straight domain boundaries. New striped surface phases were also observed, including a 13 x radical3 phase with a density that was 69% of the close-packed density; these striped phases appeared topographically higher in STM images than close-packed monolayers.

Mechanism of UV Photoreactivity of Alkylsiloxane Self-Assembled Monolayers

A molecular level understanding of the photoreactivity of self-assembled monolayers (SAMs) becomes increasingly important as the spatial resolution starts to be limited by the size of the resist and the spatial extent of the photochemical reactions in photoresist micropatterning. To this end, a number of surface characterization techniques were combined to understand the reactive agents, reactive sites, kinetics, and reaction pathways in the UV photoreactivity of octadecylsiloxane (ODS) SAMs. Quantitative analysis of our results provides evidence that ground state atomic oxygen is the primary reactive agent for the UV degradation of ODS SAMs. UV degradation, which follows zero-order kinetics, results in the scission of alkyl chains instead of the siloxane headgroups. Our results suggest that the top of the ODS SAMs is the preferential reactive site. Using a novel, highly surface sensitive technique, fluorescence labeling of surface species, we identified the presence of submonolayer quantities chemical functional groups formed by the UV degradation. These groups are intermediates in a proposed mechanism based on hydrogen abstraction.

Modification and Stability of Aromatic Self-Assembled Monolayers upon Irradiation with Energetic Particles

We have studied ion and electron irradiation of self-assembled monolayers (SAMs) of 2-(4'-methyl-biphenyl-4yl)-ethanethiol (BP2, CH3-C6H4C6H4CH2CH2-SH), phenyl mercaptan (PEM, C6H5CH2CH2-SH), and 4'-methyl-biphenyl-4-thiol (BP0, CH3-C6H4C6H4-SH) deposited on Au(111) substrates. Desorption of neutral particles from PEM/Au and BP2/Au was investigated using laser ionization in combination with mass spectrometry. The ion-induced damage of both BP2 and PEM SAMs is very efficient and interaction with a single ion leads to the modification of tens of molecules. This feature is the result of a desorption process caused by a chemical reaction initiated by an ion impact. Both for ions and electrons, experiments indicate that the possibility for scission of the Au-S bond strongly depends on the chemical nature of the SAM system. We attribute the possible origin of this effect to the orientation of the Au-S-C angle or adsorption sites of molecules. The analysis of electron-irradiated PEM/Au and BP2/Au, using ion-initiated laser probing, enabled measurements of the cross section for the electron-induced damage of the intact molecule or specific fragment. Analysis of electron-irradiated BP0/Au by using time-of-flight secondary ion mass spectrometry (TOF-SIMS) provides direct evidence for the quasi-polymerization process induced by electron irradiation.

Formation of alkanethiol monolayer on Ge(111)

Alkanethiols, CH3(CH2)(n-1)SH, are shown to react readily with HF-treated Ge(111) surface at room temperature to form a high-quality monolayer. The resulting films are characterized by using contact angle analysis (CAA), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE), and high-resolution electron energy loss spectroscopy (HREELS). The octanethiol and octadecanethiol films on Ge(111) both exhibit 101 degrees and 40 degrees water and hexadecane contact angles, respectively. These values indicate that the thiol surface coverage is relatively high, and that the films possess a high degree of orientational ordering. The angle-resolved XPS analysis supports that thiols are bound to the Ge surface by Ge-S bonds at the monolayer/Ge interface. The film thickness values obtained by XPS and SE agree well with the earlier reported values on alkyl monolayers on Ge(111) prepared by Grignard reaction. On the basis of HREEL spectra taken after thermal annealing steps, the monolayers are found to be thermally stable up to 450 K. The thermal stability provides further evidence that thiols are covalently bonded to Ge(111).