: Modification of closely spaced (approv 1.5 micro m) microelectrodes (approv 2.5 micro m wide x 50 micro m long x 0.1 micro m high) with electroactive materials gives rise to a variety of new kinds of 'electronic devices'. Surface modification of microelectrode arrays with metal oxides, e.g. WO3, conventional redox polymers, e.g. viologen-based polymers, and so-called conducting redox polymers, e.g. poly(3-methylthiophene), allows demonstration of a variety of microelectrochemical 'transistors' having properties that depend on the electroactive material. While the response time of such devices is slow by solid state electronics standards, chemically sensitive transistors can be envisioned as sensitive, specific sensors. Solid state microelectrochemical devices can be made that involve the use of ionic conducting polymers as the 'solvent'/electrolyte system. Solid state microelectrochemical transistors can be designed to be sensitive to certain gases.

In their article, Cheng and colleagues present the plan for reopening colleges and universities in Taiwan. There are important differences between Taiwan and other countries, but residential colleges and universities present similar challenges to pandemic control for all. Considering how well Taiwan has managed COVID-19 overall, the editorialists believe that the plan for safely reopening colleges and universities in Taiwan offers important principles.

ADVERTISEMENT RETURN TO ISSUEPREVEditorialNEXTThe Influencers: Mentors Can Make a Difference in Your CareerMark S. Wrighton​Mark S. WrightonWashington University in St. Louis, St. Louis, Missouri 63130, United StatesE-mail: [email protected]More by Mark S. WrightonCite this: ACS Cent. Sci. 2019, 5, 9, 1477–1478Publication Date (Web):September 25, 2019Publication History Published online25 September 2019Published inissue 25 September 2019https://doi.org/10.1021/acscentsci.9b00841Copyright © 2019 American Chemical SocietyRIGHTS & PERMISSIONSACS AuthorChoiceArticle Views2059Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Get e-AlertsSUBJECTS:Amorphous materials,High school teachers,Photochemistry,Post-secondary education,Students,Teaching and learning methods Get e-Alerts

The electrochemical behavior of single-crystal p-type GaAs in aqueous solutions containing several redox couples (13-/1-, Fe(III)/Fe(II), Sn(IV)/Sn(II), Eu(III)/Eu(II)) in the dark and under irradiation is described. The observation that the difference in potential between that for the onset of photocurrent and the standard potential for the redox couple was 0.40.5 V, independent of the couple, leads to a revised model for semiconductor/electrolyte solution interface with semiconductors having a high density of surface states with energies within the band-gap region. In such a surface controlled system the Fermi level of the semiconductor is pinned at the surface state level. Several solar cells in which p-GaAs shows stable behavior are described. The cell p-GaAs/I3-(0.25 M),I-(0.75 M)/Pt showed an open-circuit voltage of 0.20 V and a short-circuit current density of 30 mA/cm2 under irradiation with 1.7" He-Ne laser. The quantum efficiency at the maximum photocurrent in this cell was about 95%.

Microfabrication techniques have been used to prepare electrode surfaces reproducibly with well-defined composition and active area. Characterization of surfaces thus prepared leads to straightforward composition-activity and structure-activity relations. Conventional e(-) beam deposition/lift-off techniques were used to fabricate the catalysts from a photopatterned resist on Pt. Catalysts consist of an array of closely spaced microstructures (circles or squares, 0.1 mum thick, 10-200 mum wide) of Sn on Pt. Characterization of these structures by Auger electron spectroscopy shows that the Pt areas are relatively rich in C, whereas the Sn is relatively rich in O, before and after exposure to aqueous solutions containing the organic fuel. After 3-5 min of use in 0.5 M H2SO4/MeOH, at potentials at which fuel oxidation occurs, redistribution of Sn to the Pt regions was observed. Sn redistribution on the time scale of the experiments is inhibited by capping the Sn with a Pt layer. In samples with capped Sn squares, the length of the Pt-Sn contact line was varied by changing the size of the structures while the exposed area of the electrode was kept constant, The activity of the capped Pt-Sn structures depends linearly on the Pt-Sn contact edge length. Thus, it is possible to confine the catalytic area of Pt-Sn electrodes by working with the appropriate structure design. The catalytic area of microfabricated Pt-Sn is located in the zone of intimate contact between the Pt and the Sn layers.

The electrocatalytic oxidations of water-soluble alcohols and diols and some other small organic molecules (MeOH, EtOH, 2-fluoroethanol, 2,2-difluoroethanol, 2,2,2-trifluoroethanol, CH2(OH)(2), ethylene glycol, n-PrOH, i-PrOH, 1,4-butanediol, 1,3-propanediol, 1,2-propanediol, t-BuOH, neopentyl alcohol, benzyl alcohol, 2-Me-1-PrOH, formic acid, acetaldehyde, acetic acid, propionaldehyde, propionic acid, acetone, glycolaldehyde, glyoxal, glycolic acid, glyoxylic acid, oxalic acid) have been compared in aqueous 0.5 M H2SO4 at C and Au electrodes modified with an electrodeposited Pt-Sn catalyst at low (-0.1 to 0.5 V vs SCE) potentials. The Pt-Sn catalyst is electrochemically deposited and forms as a smooth deposit on Au electrodes. At a catalyst loading of similar to 0.5 mg/cm(2) the electrodeposition of the Pt-Sn catalyst results in formation of adherent similar to 0.8 mu m size particles on C electrodes. In general, for organic molecules containing only C, H, and O with two or more carbon atoms, the presence of H atoms on both the alpha- and beta-carbon results in a relatively negative potential for onset of catalytic current (usually between -0.1 and -0.2 V vs SCE) at Pt-Sn compared to Pt alone. Of the alcohols and diols studied, formaldehyde (which exists in aqueous solutions as the hydrated form, CH2(OH)(2)) shows the highest electrocatalytic currents at the Pt-Sn catalyst. The ultimate product, via HCOOH, is CO2. EtOH is oxidized only to acetic acid on Pt-Sn electrodes. The EtOH and n-PrOH oxidation yields, determined by exhaustive electrolysis, are 4 e(-) per molecule, while i-PrOH yields only 2 e(-) per molecule. We confirmed the generation of acetic acid, propionic acid, and acetone as the final products for the oxidations of EtOH, n-PrOH, and i-PrOH, respectively, by C-13 NMR and/or GCMS. The electron yield of the oxidation of ethylene glycol at Pt-Sn surfaces is only 4 e(-) per molecule instead of the value of 8 e(-) per molecule expected for the oxidation of ethylene glycol to oxalic acid. Glycolic acid (CHOCOOH) is the oxidation product by GCMS. This substance is not electrocatalytically oxidized on Pt-Sn at potentials negative of similar to +0.4 V vs SCE in comparison to the -0.1 V vs SCE onset for ethylene glycol oxidation. MeOH, the molecule with the highest electron yield on Pt-Sn (6 e(-) per molecule), unfortunately shows the most positive potential onset for oxidation among the group of alcohols compared in this study, while the two intermediates along the path of oxidation to CO2, formaldehyde, and formic acid are oxidized on Pt-Sn at very negative potentials compared to MeOH. Thus, the conversion of MeOH to formaldehyde is the efficiency-determining step in the oxidation of MeOH to CO2.

A blue electroluminescent polymer containing 9,10-diphenylanthracenevinylenephenylene and alkylene block in the main chain was prepared by the Wittig reaction. A singlelayer electroluminescent device was fabricated by spin casting a solution of the novel polymer onto glass substrates coated with a pattern indium tin oxide (ITO). A bluish white luminescence was obtained from the device using aluminum as the cathode contact with intensities in the range of 45-80 cd/m(2) and external quantum efficiency of 0.01%.

The title compound, a potential ligand for transition metals, was prepared by coupling of two 4,7-phenanthroline-5,6-dione molecules in presence of ammonia under reductive conditions. A straightforward synthesis of 4,7-phenanthroline-5,6-dione is also presented.

This paper describes the electrochemical characterization of electrode-confined siloxane polymers that contain both naphthoquinone (NQ) and benzylviologen (BV2+) subunits. These ''homopolymers,'' abbreviated (NQ-BV3+)(n) and (NQ-BV-BV5+)(n), are derived from monomers, 2-chloro-3-[[2-{dimethyl[[[[N'-[[4-(trimethoxysilyl)- phenyl]methyl]-4,4'-bipyridiniumyl]methyl]phenyl]methyl]ammonium}ethyl]amino]-1,4-naphthoquinone, 1a, and 2-chloro-3-[[2-{dimethyl[[[[[[[[N'-[N'-[[4-(trimethoxysilyl)phenyl]methyl]-4,4'bipyridiniumyl]methyl]- phenyl]methyl]-4,4'-bipyridiniumyl]methyl]phenyl]methyl]ammonium}ethyl]amino]-1,4-naphthoquinone, 2a, respectively. Particular to these types of surface-confined homopolymers is the ability to ''trap'' charge at low pH in the form of reduced quinone. Between pH 10.0 and 7.0, (NQ-BV3+)(n) and (NQ-BV-BV5+)(n) undergo reversible 3e-/2H(+) and 4e-/2H(+) reduction, respectively, consistent with 2e-/2H(+) reduction of NQ subunits to the hydroquinone NQH(2) and 1e- reduction of BV2+ subunits to the radical cation BV+.. Below pH 6, however, NQ/NQH(2) reduction becomes irreversible in both polymers, whereas BV2+/+ reduction remains reversible. Electrochemical irreversibility of NQ/NQH(2) in these polymers occurs because its electrochemistry is mediated by the BV2+/+ couple. Between pH 10.0 and 7.0, BV2+/+ can mediate both reduction and oxidation of NQ/NQH(2), whereas below pH 6, the thermodynamics are such that BV2+/+ can only mediate the reduction of NQ. This behavior is similar to that of a previously studied (benzylviologen)-benzoquinone-(benzylviologen) polymer, (BV-Q-BV6+)(n).(1) Charge in the form of reduced quinone is trapped in (BV-Q-BV6+)(n) at low pH because there is essentially no direct charge transport through the Q/QH(2) system. Charge is also trapped in high coverages of (NQ-BV3+)(n) and (NQ-BV-BV5+)(n), indicating no direct charge transport through the NQ/NQH(2) system. Unlike (BV-Q-BV6+)(n), however, monolayers comprised of 1a or 2a exhibit charge transport through the NQ/NQH(2) system. The flexibility of these monolayers apparently allows direct contact of the NQ subunit with the electrode surface. Less flexible and more robust surface-confined polymers, abbreviated (NQ-BV3+/siloxane)(n) and (NQ-BV-BV5+/siloxane)(n), can be prepared by copolymerization of 1a or 2a with 1,2-bis(trimethoxysilyl)ethane. Charge trapped in (NQH(2)-BV3+/siloxane)(n) or (NQH(2)-BV-BV5+/siloxane)(n) can be released and delivered to the surface of the electrode via chemical mediation or by an increase in solution pH. For example, the redox couple I-3(-)/I- will catalytically release the trapped charge when the potential of the electrode is brought close to E(o)'(I-3(-)/I-). Surfaces modified with (NQ-BV3+/siloxane)(n) or (NQ-BV-BV5+/siloxane)(n), however, are impermeable to the large anionic redox couple Fe(CN)(6)(3-/4-), preventing mediated charge release by this reagent.The lack of electrostatic binding of Fe(CN)(6)(3-/4-) to electrodes modified with (NQ-BV3+/siloxane)(n) or (NQ-BV-BV5+/siloxane)(n) suggests a high degree of crosslinking in these polymers provided by 1,2-bis(trimethoxysilyl)ethane. At neutral pH, dioxygen will chemically induce the release of charge trapped in (NQH(2)-BV3+/siloxane)(n) or (NQH(2)-BV-BV5+/siloxane)(n). The irreversible production of H2O2 upon oxidation of NQH(2) in water, however, prevents the return of charge to the electrode. Charge release is also demonstrated by pH jump experiments where an increase in pH shifts E(o)'(NQ/NQH(2)) to a potential where BV2+/+ mediate the oxidation of NQH(2) to NQ and deliver charge to the electrode.

The feasibility of preparing multilayer porphyrin films with monolayer control of composition and thickness is demonstrated. Successive dip-coating of tetraruthenated zinc porphyrin, [ZnTPyPBpy](4+), and mesotetraphenylporphyrin sulfonate, [M-TPPS](4-), is shown to result in linear growth of the film thickness (12.7(6) Angstrom/bilayer) and optical absorbance as a function of the number of bilayers. These results strongly suggest a layer-by-layer assembly of compositionally homogeneous films of up to 30 bilayers. The assembled multilayer composite films have been characterized by UV-vis spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, atomic force microscopy, and electrochemical methods. Electrodes modified with these films exhibit a reversible wave at 0.94 V and are photocatalytically active toward the reduction of O-2. Our findings suggest that the layer-by-layer growth method may be a general route to compositionally modulated porphyrin films of arbitrary thickness and rationally tailored catalytic and photophysical properties.

A photosensitive self-assembled monolayer (SAM) is selectively irradiated to fabricate a pattern on an Au electrode, and a thin film of aniline or 3-methylthiophene is deposited on it by electropolymerization. Adhesion of the polymer films can be controlled by the monolayer terminal group. Applying tape to the sample and peeling it away selectively removes the conducting polymer film to the tape in a near-micron resolution pattern.

A series of novel poly(2,5-dialkoxy-p-phenyleneethynylene) (PPEs) has been prepared by the palladium-catalyzed cross-coupling polycondensaton of aryl acetylenes and aryl iodides. Different alkoxy side chains including n-hexadecyloxy, n-octyloxy, (2-ethylhexyl)oxy, (2-methylpropyl)oxy, (3-(dimethylamino)propyl)oxy, and (7-carboxyheptyl)oxy groups were attached to the rigid-rod polymer main chain. With this structural concept, polymers having an identical conjugated backbone but different supramolecular structures in the solid state could be achieved. X-ray diffraction measurements on thin films show that the polymers which have sterically hindered side chains are essentially disordered, while those with only linear side chains can form lamellar structures with significant degrees of long-range order. High photoluminescence (PL) quantum yields, up to 0.86 in solution and 0.36 in the solid state, have been measured for the new materials. While the solution quantum yields are independent of the functionalization, solid-state quantum efficiencies were found to be related to the degree of long-range order in the samples, decreasing with increasing order. The coplanar orientation of the conjugated polymer backbones is assumed to lead to the formation of excimer complexes which provide nonemissive decay channels for the excited states. These nonemissive orientations are more significant in the materials having a greater degree of long-range order.

In an effort to better understand the relationship between molecular structure and photophysical properties, we have prepared and investigated a series of novel poly(2, 5-dialkoxy-p-phenyleneethynylene)s. Wide angle X-ray diffraction measurements show that the supramolecular structure can be easily and significantly influenced by the nature of substituents covalently linked to the rigid-rod polymer main chains. Polymers which have sterically hindered side chains are essentially amorphous, while those with only linear side chains can form lamellar structures with a significant degree of long-range order. High photo-luminescence quantum yields, up to 0.86 in solution and 0.36 in the solid state, have been measured. While the solution quantum yields are independent of the functionalization, solid state quantum efficiencies were found to be related to the degree of long-range order in the samples. In samples with a high degree of long-range order, the close proximity of the co-planar oriented polymer backbones is assumed to lead to the formation of eximer complexes which provide non emissive decay channels and, hence, result in comparable low photo-luminescence quantum yields. In samples that adopt only a small extent of long-range order, the rigid-rod conjugated polymer backbones behave as if they were ‘dissolved’ in a hydro-carbon solvent and consequently high quantum efficiencies are obtained. Preliminary results indicate the suitability of these polymers as the emitting layer in electroluminescent devices.

The reaction of electrode-confined polyaniline with trifluoroacetic anhydride in acidified acetonitrile giving insulating and electroinactive trifluoroacetylated polyaniline has been studied by electrochemistry, reflectance IR, and microelectrochemistry. Variation of electrochemical. potential from 0.2 V (reduced, most reactive) to 0.6 V (oxidized by 0.5 electron per repeat unit, unreactive) vs SCE allows control of the reaction rate. Reaction of trifluoroacetic anhydride with aniline oligomers N-phenylphenylenediamine and N,N'-diphenylphenylenediamine gave N-trifluoroacetylation products exclusively, exhibiting positive shifts in oligomer oxidation potential of >0.5 V, with terminal amines reacting considerably faster than internal amines. Reflectance IR following the potential-dependent growth of CO and CS peaks for macroelectrode films of polyaniline treated with trifluoroacetic anhydride showed similar potential dependence of reactivity as conductivity measurements during trifluoroacetylation of polyaniline-derivatized microelectrode arrays. Polyaniline trifluoroacetylation was accompanied by narrowing but no shifting of the potential window of electroactivity and conductivity, and eventual elimination of all conductivity. Trifluoroacetylation of polyaniline terminal amines, rapid at all potentials, does not detectably affect conductivity. Also examined by electrochemistry were the reactions of polyaniline with other anhydrides resulting in the reactivity order (F3CCO)(2)O > (Cl3CCO)(2)O > (H2ClCCO)(2)O > (HCl2CCO)(2)O >> (H3CCO)(2)O. IR through polyaniline electrodeposited onto optically transparent Au electrodes shows that essentially complete loss of polyaniline electroactivity occurs when approximate to 25% of nitrogens are trifluoroacetylated. Electroactivity and conductivity of trifluoroacetylated polyaniline may be recovered by hydrolysis in K2CO3/CH3OH/O-2 solution to regenerate polyaniline. Use of the reversible trifluoroacetylation of polyaniline provides a proof-of-concept for a new approach to an erasable-programmable-read-only-memory device.

Chemical force microscopy (CFM) has been used to measure adhesion and friction forces between probe tips and substrates covalently modified with self-assembled monolayers (SAMs) that terminate in distinct functional groups. Probe tips have been modified with SAMs using a procedure that involves coating commercial Si3N4 cantilever/tip assemblies with a thin layer of polycrystalline Au followed by immersion in a solution of a functionalized thiol. This methodology provides a reproducible means for endowing the probe with different chemical functional groups. The spring constants and radii of the chemically modified cantilever/tip assemblies have been characterized to allow for quantitative friction and adhesion measurements. Au-coated Si and Si substrates have been treated with functionalized thiols and silanes, respectively, to produce SAM coated substrates terminating with different functional groups. A force microscope has been used to characterize the adhesive interactions between probe tips and substrates that have been modified with SAMs which terminate with COOH, CH3, and NH2 functional groups in EtOH and H2O solvents. Force vs distance curves recorded under EtOH show that the interaction between functional groups decreases as follows: COOH/COOH > CH3/CH3 > COOH/CH3. The measured adhesive forces were found to agree well with predictions of the Johnson, Kendall, and Roberts (JKR) theory of adhesive contact and thus show that the observed adhesion forces correlate with the surface free energy of the molecular groups in EtOH. Electrostatic contributions to adhesive forces have also been studied using a COO-/NH3+ tip/surface in aqueous solution. Force vs distance curves recorded as a function of ionic strength show that the observed adhesive interaction decreases with increasing ionic strength. These results have been interpreted in terms of contact and noncontact contributions to the experimentally measured adhesive force. The friction forces between tips and samples modified with COOH and CH3 groups have also been measured as a function of applied load. The magnitude of the friction force was found to decrease in the following manner with different tip/sample functionalities: COOH/COOH > CH3/CH3 > COOH/CH3. Friction forces between different chemical functional groups thus correlate directly with the adhesion forces between these same groups. Specifically, high friction is observed between groups that adhere strongly, while low friction is observed between weakly interacting functional groups. The dependence of friction forces on the tip and sample functionality is shown to be the basis for chemical force microscopy in which lateral force images are interpreted in terms of the strength of both adhesive and frictional interactions between different functional groups.

Solution-cast films of four different poly(2,5-dialkoxy-p-phenyleneethynylene) molecules, I-IV, with varying backbone chain lengths and varying alkoxy substituent chain lengths, and an alternating copolymer of 1,4-diethynyl-2,5-dihexadecyloxybenzene and 9,10-dibromoanthracene, V, have been characterized electrochemically and by X-ray diffraction (XRD) and differential scanning calorimetry (DSC), XRD and DSC show that the polymers have varying degrees of order and crystallinity based on long-range lamellar structure, Cyclic voltammetry in liquid SO2/electrolyte shows that the onset of oxidation for I-IV occurs at similar to 1.05 V vs SCE with the more crystalline polymers having slower electrochemical response than the less crystalline ones, In situ characterization of the potential dependence of conductivity in the same medium shows that the maximum conductivities of I-IV range from similar to 0.2 to similar to 5 Ohm(-1) cm(-1), suggesting that higher conductivity is associated with lower long-range order in the polymer films but showing little dependence on average polymer chain length, I-IV all have maximum conductivity at similar to 1.6 V vs SCE and finite potential windows of high conductivity similar to 0.55 V wide, indicating that the potential of maximum conductivity and the width of the window of high conductivity are determined by molecular rather than bulls. properties, For V, the onset of oxidation occurs at similar to 0.8 V vs SCE, the potential of maximum conductivity is similar to 1.5 V vs SCE, and the width of the potential window of high conductivity is similar to 0.85 V.

Upon electropolymerization of the corresponding monomer, patterns of polyaniline, poly(3-methyl-thiophene), or polypyrrole replicate the pattern formed by selective irradiation of a photosensitive monolayer confined to an electrode surface. Irradiation of monolayers of bis[11-(4-azidobenzoyl)oxy]-1-undecyl disulfide, I, on Au in the presence of various primary or secondary amines results in the attachment of the amine in very high yields. Irradiation of Au-I substrates through a mask results in a patterned monolayer. Electrodeposition of aniline from aqueous solution, or 3-methylthiophene or pyrrole from CH3CN, results in polymer deposition preferentially on the nonirradiated regions of the surface. Polymer patterns are characterized by optical microscopy and by stylus profilometry. Cyclic voltammetry and X-ray photoelectron spectroscopy are used to measure deposition contrast and show that under certain electrochemical conditions the ratio of polymer deposition on nonirradiated monolayer regions to irradiated monolayer regions is greater than 1000:1. Cyclic voltammetry of the monomers, Fe(CN)(6)(3-/4-), and N,N,N',N'-tetramethyl-1,4-phenylenediamine(2+/+/0) shows that electron transfer rates are attenuated on monolayers of Au-I compared to bare Au and are decreased further on monolayers of I irradiated in [CH3(CH2)(3)]2NH or [CH3(CH2)(7)]2NH. Differences in electron transfer through these monolayers agree with the polymer patterning results and are shown to be the principal reason for selective polymer deposition on these photopatterned monolayer substrates.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTUse of the Redox-Active Ligand 1,1'-Bis(diphenylphosphino)cobaltocene To Reversibly Alter the Rate of the Rhodium(I)-Catalyzed Reduction and Isomerization of Ketones and AlkenesIvan M. Lorkovic, Ronald R. Duff Jr., and Mark S. WrightonCite this: J. Am. Chem. Soc. 1995, 117, 12, 3617–3618Publication Date (Print):March 1, 1995Publication History Published online1 May 2002Published inissue 1 March 1995https://pubs.acs.org/doi/10.1021/ja00117a033https://doi.org/10.1021/ja00117a033research-articleACS PublicationsRequest reuse permissionsArticle Views1439Altmetric-Citations157LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts

Electron beam deposition ~EBD! is a maskless technique suitable for the fabrication of nanometer scale structures. Metals can be deposited from an organometallic gas, but simultaneous carbon deposition typically yields grossly impure ~;25% metal! deposits. We have found that the metal content of the deposited solid is dramatically improved by performing the whole EBD process in a reactive gaseous environment containing a source of oxygen ~O2 or H2O! in addition to the organometallic gas. With simple procedures we prepared Au deposits showing significantly diminished C content ~up to 50% metal! as the partial pressure of O2 ~or H2O! is increased in the gas.© 1995 American Institute of Physics. Electron beam deposition ~EBD! 1,2 provides an inexpen- sive way to create features of different sizes, shapes, and materials in the submicron or nanometer scale. The focused e 2 -beam of a scanning electron microscope ~SEM! has been shown to cause the growth of dots, columns ~typically less than 100 nm diam!, lines or thin films, if an appropriate gas is present in the vicinity of the substrate. Some or all of the gas molecules that adsorb on the area that is irradiated by the electrons are dissociated, and a deposit is formed consisting of the nonvolatile remains from that dissociation. Metallic nanostructures can be created by using an organometallic precursor gas. 3-7 In previous EBD work, through, the metal- lic purity of the deposits has been limited by a substantial amount of simultaneously deposited carbon, coming both from the residual hydrocarbon contamination gases inside the SEM chamber and from the organometallic precursor compound itself. Here we present a method that significantly reduces car- bon incorporation in EBD. Our method is based on the si- multaneous presence of two gases during the deposition pro- cess: the organometallic precursor gas and an ''environmental gas.'' Since, in our case, the partial pressure of organometallic gas is constant and around two orders of magnitude smaller than that of environmental gas, we denote the measured total pressure as environmental pressure Penv . In our experiments the environmental gas is either H2O vapor, Ar, or a mixture of 80% Ar and 20% O2 ~Ar/O2!. We found that the presence of Ar/O2 or H2O reduces the C content of the deposits whereas Ar does not. Although no precautions were taken to reduce the residual contamina- tion in the SEM chamber, nor to carefully handle or clean the sample, we obtained Au contents as high as 50%. Instead of a conventional SEM we used an environmen- tal SEM ~ESEM! for the EBD process. 8 The ESEM allows imaging in the presence of up to 20 Torr of certain gases. Ionization of gas molecules above the surface allows the de- tection of secondary electrons for imaging. Here we show that this highly ionized gas, which constitutes our environ- mental gas, can also create a reactive environment which reduces the C content of Au deposited by EBD. In addition, we simplified the usual subchamber setup for EBD. Gener- ally, the sample is placed inside a subchamber within the SEM chamber to allow the presence of a gas around the sample without breaking the SEM vacuum. Instead of feed- ing the precursor gas into the subchamber from an external source, we put a small quantity of the organometallic com- pound in a built-in reservoir inside the subchamber ~see Fig. 1!. The reservoir communicates with the subchamber through a fixed 50-mm aperture and the sample is accessed by the e 2 -beam through a fixed 1-mm aperture. We could not, therefore, vary the partial pressure of the organometallic gas, estimated to be ;130 mTorr above the substrate by comparing our growth rates with those reported in Ref. 6. Its exact value does not affect our findings, since it is constant while we vary Penv . Accurate control of the growth rate would require variable apertures, pressure gauges, and an electron dose gauge, as reported by others. 5,6 Our efforts were directed toward improving the composition of the Au deposits by judicious choice of the environmental gas ~Ar, Ar/O2 ,o r H 2 O !and its partial pressure ~;0-10 Torr!. The substrate was mounted close to the top aperture of the sub-