Article pubs.acs.org/Langmuir
Covalent Attachment of Diamondoid Phosphonic Acid Dichlorides to Tungsten Oxide Surfaces Fei Hua Li,† Jason D. Fabbri,† Raisa I. Yurchenko,‡ Alexander N. Mileshkin,‡ J. Nathan Hohman,† Hao Yan,† Hongyuan Yuan,† Ich C. Tran,§ Trevor M. Willey,§ Michael Bagge-Hansen,§ Jeremy E. P. Dahl,† Robert M. K. Carlson,† Andrey A. Fokin,‡,∥ Peter R. Schreiner,∥ Zhi-Xun Shen,† and Nicolas A. Melosh*,† †
Geballe Laboratory for Advanced Materials, Stanford University, 476 Lomita Mall, Stanford, California 94305, United States Department of Organic Chemistry, Kiev Polytechnic Institute, pr. Pobedy 37, 03056 Kiev, Ukraine § Materials Science and Technology Division, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550, United States ∥ Institute of Organic Chemistry, Justus-Liebig University, Heinrich-Buff-Ring 58, D-35392 Giessen, Germany ‡
S Supporting Information *
ABSTRACT: Diamondoids (nanometer-sized diamond-like hydrocarbons) are a novel class of carbon nanomaterials that exhibit negative electron affinity (NEA) and strong electron− phonon scattering. Surface-bound diamondoid monolayers exhibit monochromatic photoemission, a unique property that makes them ideal electron sources for electron-beam lithography and high-resolution electron microscopy. However, these applications are limited by the stability of the chemical bonding of diamondoids on surfaces. Here we demonstrate the stable covalent attachment of diamantane phosphonic dichloride on tungsten/tungsten oxide surfaces. Xray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FTIR) spectroscopy revealed that diamondoid-functionalized tungsten oxide films were stable up to 300−350 °C, a substantial improvement over conventional diamondoid thiolate monolayers on gold, which dissociate at 100−200 °C. Extreme ultraviolet (EUV) light stimulated photoemission from these diamondoid phosphonate monolayers exhibited a characteristic monochromatic NEA peak with 0.2 eV full width at half-maximum (fwhm) at room temperature, showing that the unique monochromatization property of diamondoids remained intact after attachment. Our results demonstrate that phosphonic dichloride functionality is a promising approach for forming stable diamondoid monolayers for elevated temperature and highcurrent applications such as electron emission and coatings in micro/nano electromechanical systems (MEMS/NEMS).
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INTRODUCTION Higher diamondoids are a novel class of well-defined nanomaterials comprised of multiple units of the diamond cage structure.1,2 Like hydrogen-terminated diamond, these molecules exhibit negative electron affinity (NEA),3,4 leading to highly monochromatic electron photoemission when attached to gold or silver surfaces.5−7 Surprisingly, these monolayer thick films exhibit an order of magnitude higher electron emission efficiency and intensity NEA peak than a state-of-the-art diamond film, while retaining sub-300 mV electron energy distributions.5 This property is useful in electron emission applications that benefit from a narrow electron energy bandwidth.8−10 To realize this unique application, robust attachments of diamondoids to cathode surfaces are necessary. Earlier studies of higher diamondoids explored the electronic structure of single-molecule tetramantane diamondoids thermally evaporated onto Au (111) without chemical bonding between diamondoids and substrate.11 These evaporated © 2013 American Chemical Society
diamondoid molecules exhibited long-range order, implying high molecular mobility and weak interactions between diamondoids and the substrate. This enabled diamondoids to be manipulated by STM for direct investigation of molecular orbitals and electron−vibrational coupling of higher diamondoids, revealing the spatial correlation in its electronic structure and electron−vibrational coupling. Applications of diamondoids are enhanced by the ability to functionalize these molecules,5,12 opening up a myriad of possibilities by combining these molecules with other systems, including fixing them onto solid substrates for applications in electron emission. Previously, we have demonstrated that thiol-functionalized diamondoids can form self-assembled monolayers (SAMs) on gold and silver surfaces.12,13 With these SAMs, we observed a Received: May 13, 2013 Revised: July 9, 2013 Published: July 15, 2013 9790
dx.doi.org/10.1021/la401781e | Langmuir 2013, 29, 9790−9797
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high-yield monochromatic photoemission as a consequence of the NEA and strong electron−phonon scattering in diamondoids.6 The thiolate bond may also participate in efficient electron coupling between the diamondoid and the metal substrate (electron reservoir) and contribute to the sharp NEA peak.14 While diamondoid-based SAMs offer intrinsic advantages over bulk NEA materials as electron emitters in terms of electron conduction and narrow energy distribution in their molecular electronic states, the low stability of diamondoid− substrate attachment impedes the realization of diamondoidbased electron emitters. Although thiol-functionalized diamondoids form crystalline, complete, and uniform monolayers on metal substrates, the lability, thermal instability, and oxidative sensitivity of thiol−metal attachments make these monolayers undesirable for long-term device applications.5,15−17 Extensive work has been conducted on the selective derivatization of diamondoids with various functional groups, including hydroxyls, carboxylic acids, alkenes, thiols, halides, silanes, and amines.18 In addition to thiols, we have explored the attachment of diamondoid−alkenes and alkynes on hydrogen-passivated silicon surfaces via a method developed by Chidsey et al.19 However, the need for neat diamondoid alkene or alkyne derivatives and preventing oxidation of the silicon in dilute solutions of diamondoid alkene and alkyne derivatives makes these types of attachment rather challenging. Here we demonstrate robust covalent attachment of diamondoids to tungsten oxide via a dichlorophosphoryl functionality.20 Tungsten is an ideal material for use in electron emitters due to its electrical and thermal stability. Oxidation of the tungsten surface provides hydroxyl functionality that can react with phosphonic dichlorides to form covalently attached diamondoids tethered by phosphonate linkers. We expect the P−O bonds (3.5 eV/bond) immobilizing the diamondoids on tungsten/tungsten oxide to be twice as strong as S−Au attachments (1.7 eV/bond), providing a more robust attachment of diamondoids to surfaces.21,22 In addition, this technique may enable attachment of geometrically diverse diamondoids onto a variety of oxidized substrates and could be applied to nonplanar substrates used in cathode electron emitters.23−32
Scheme 1. Proposed Reaction for the Synthesis and Attachment of 1-Dichlorophosphoryldiamantane (1) to a Tungsten Oxide Surface
hydroxyl groups.36,37 We immersed substrates in a toluene solution of 1 for 2 days to coat the tungsten oxide with diamondoids. Condensation between phosphonic dichloride and the metal oxide surface hydroxyl eliminates HCl and forms covalent bonds between the phosphonate and the substrate.38 Alternatively, exposure of the solution to water can displace the chlorine, yielding phosphonic acid, which can similarly react with surface hydroxyl to form a molecular attachment.26,29,39,40 Both reaction pathways yield surface-bound diamondoid molecules on tungsten oxide (Scheme 1). Diamondoid monolayers were characterized using FTIRATR (Figure 1). The C−H bonds terminating the diamondoid
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RESULTS AND DISCUSSION Direct phosphorylation of diamantane yields a mixture of apical monodiamantylphosphonic and didiamantylphosphinic acid chlorides.20 For the selective preparation of the medial diamantane phosphonic dichlorides (1), we utilized two different procedures previously developed for adamantane, namely phosphorylation in either sulfuric33 or trifluoroacetic acids (Scheme 1).34 Utilizing 1-diamantyl derivatives 2 (X = Br or OH) prepared in accordance with our previously developed procedures,35 target 1 was obtained in high preparative yields and was characterized using nuclear magnetic resonance (NMR) spectroscopy (see Experimental Procedures and Supporting Information for details). Diamondoid (medial diamantane) monolayers were prepared by attaching 1 to tungsten oxide, and the resultant films were characterized by FTIR-ATR (Fourier-transform infrared attenuated total reflectance) and XPS (X-ray photoelectron spectroscopy). Planar silicon substrates were sputter-coated with 10 nm of titanium (adhesion layer) followed by 100 nm of tungsten. Tungsten surfaces were oxidized in oxygen plasma, yielding a several nanometer thick tungsten oxide terminated by
Figure 1. FTIR-ATR spectra showing the C−H stretch region of medial diamantane monolayers on tungsten oxide at RT and heated up to 400 °C.
exhibit strong infrared absorption between 3000 and 2800 cm−1.41 Bare tungsten oxide substrates were used for background subtraction. Absorption spectra indicate the presence of C−H groups from medial diamantane on the coated substrate. Diamondoids possess CH and CH2 groups exclusively. Methylene (CH2) asymmetric and symmetric absorption modes are observed at 2918 and 2862 cm−1, respectively.15,42−45 The weak absorption present as a shoulder at 2931 cm−1 is likely attributed to Fermi resonance between 9791
dx.doi.org/10.1021/la401781e | Langmuir 2013, 29, 9790−9797
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the methylene band and its overtone.41 Methine (CH) absorption occurs at 2889 cm−1. Methylene absorption is sensitive to conformational order within the SAM, with highly ordered monolayers at the lower end of the wavenumber range (ca. 2910 cm−1).46 The methylene absorption observed at 2918 cm−1 is consistent with a low-order film, which is expected due to the strength of the phosphonate bond that prevents rearrangement within the monolayers to form crystalline packing. The absence of methyl (CH3) stretches at 2862− 2882 and 2952−2972 cm−1 confirms that the C−H absorptions come from diamondoids rather than adventitious or solvent sources for carbon.46 The stability of the diamondoid attachment to tungsten oxide was assessed from thermal stability measurements using temperature-dependent FTIR and XPS. Samples for FTIR testing were prepared by resistively heating the tungsten sample inside a vacuum chamber (10−8 Torr), removed, and rapidly transferred into a dry air-purged FTIR. Detachment of diamondoids from the tungsten oxide surface was observed between 300 and 350 °C, marked by the disappearance of the C−H absorption peaks in FTIR (Figure 1). This is a substantial improvement over conventional diamondoid thiolate monolayers on gold, which dissociate at 100−200 °C.47,48 The temperature-dependent detachment was confirmed by in situ variable-temperature XPS. Carbon composition, extracted from the C 1s peak, was monitored during substrate heating within the XPS chamber. The initial carbon atomic percentage observed for diamondoid monolayers on tungsten oxide was 37%, which dropped significantly between 300 and 400 °C (Figure 2a, blue dots), in agreement with temperature-
to 3.6 eV/bond for C−C and 4.3 eV/bond for C−H);50 therefore, the desorption of diamondoids from the surface is more likely to occur via the detachment of diamondoid molecules from the phosphonate linker than by cleavage of the P−O bond (3.5 eV/bond), which is more stable. Thermal stability below the desorption threshold was assessed by holding the temperature at 250 °C and monitoring the change in carbon composition over time. After a small initial decrease attributed to desorption of adventitious carbon (∼5%), the carbon coverage remained stable for 3 h (Figure 2b, blue circles). For comparison, adventitious carbon on bare tungsten oxide surfaces desorbed within 15 min at 250 °C (green circles). High-resolution XPS scans of the phosphorus (P 2p) binding energy confirmed phosphorus functionalization of the surface, attributed to covalently attached phosphonate linker with the diamondoid moiety. The observed chemical shift for P 2p agrees with literature values for organophosphonate monolayers bonded to metallic substrates.27,44 High-resolution XPS over the binding energies of chlorine (Cl 2p) revealed a weak signature attributed to physisorbed 1. The physisorbed coverage is low (
