Electron Beam Induced Surface Activation of the Metal- Organic

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C: Physical Processes in Nanomaterials and Nanostructures

Electron Beam Induced Surface Activation of the Metal-Organic Framework HKUST-1: Unravelling the Underlying Chemistry Kai Ahlenhoff, Christian Preischl, Petra Swiderek, and Hubertus Marbach J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06226 • Publication Date (Web): 31 Oct 2018 Downloaded from http://pubs.acs.org on November 12, 2018

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The Journal of Physical Chemistry

Electron Beam Induced Surface Activation of the MetalOrganic Framework HKUST-1: Unravelling the Underlying Chemistry

Kai Ahlenhoff * ǂ,1 Christian Preischl ǂ,2 Petra Swiderek*,1 Hubertus Marbach2 1 University of Bremen, Faculty 2 (Chemistry/Biology), Institute of Applied and Physical Chemistry, Leobener Straße 5, 28334 Bremen, Germany 2 Physikalische Chemie II, FAU Erlangen-Nürnberg, Egerlandstr. 3, 91058 Erlangen, Germany ǂ These authors contributed equally to this manuscript.

* Corresponding authors: University of Bremen, Institute of Applied and Physical Chemistry, Fachbereich 2 (Chemie/Biologie), Leobener Straße / NW2, Postfach 330440, D-28334 Bremen, Germany; Phone: +49 421 218 63204; email: [email protected] Germany; Phone: +49 421 218 63200; email: [email protected] ACS Paragon Plus Environment

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Abstract The local chemical activation of surface-anchored metal-organic frameworks is a novel electron beam based lithographic technique with a high potential for the fabrication of chemically and spatially well-defined nanostructures in the sub-10 nanometer regime. In this context we have performed a detailed investigation of electron beam induced surface activation (EBISA) on surface-anchored layers of HKUST-1 and copper(II) oxalate with subsequent autocatalytic growth (AG) of deposits from the precursors Fe(CO)5 and Co(CO)3NO. We use reflection absorption infrared spectroscopy (RAIRS) and measurements on electron-stimulated desorption (ESD) to identify the chemical species that trigger decomposition of the precursors on the activated surfaces. EBISA on HKUST-1 works for Fe(CO)5 but not for Co(CO)3NO and is therefore chemical selective. On the other hand, we demonstrate that copper(II) oxalate is not prone to EBISA for both precursors ruling out that Cu nanoparticles are active sites for initiating AG. A detailed discussion and comparison of the experimental results for both substrates and precursors is supported by a review of the current state of knowledge regarding electron-induced chemistry of the investigated materials and on the precursor reactivity. Based on this analysis, we discuss specific species that are likely to trigger AG following EBISA of HKUST-1.

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1. Introduction Focused electron beam induced processing (FEBIP) techniques have gained increasing attention during the last decade as a state-of-the-art tool for the controlled lithographic fabrication of structures in the nanometer regime. In FEBIP, the focused electron beam is used to very locally induce reactions of adsorbed molecules or the substrate itself to create deposits from precursor molecules. An important technique within the FEBIP family is referred to as electron beam induced deposition (EBID). It enables the fabrication of arbitrary deposit structures on surfaces from a wide range of volatile precursor molecules in a maskless writing process.1–3 EBID relies on the local electron beam induced dissociation of precursors that are typically metal-organic molecules yielding the formation of a solid deposit while the volatile fragments desorb and can thus be removed from the process. Unfortunately, the EBID process is often hampered by the unintended codeposition of carbon containing material yielding carbonaceous deposits that call for further purification steps.4 As favourable examples, quite low contamination levels have been achieved in the cases of Fe and Co deposits5 but purity depends on the preparation conditions.6 However, it was previously shown that an ultrahigh vacuum (UHV) environment is very beneficial to further enhance the deposit purity.7 In addition, UHV is particularly advantageous for autocatalytic growth (AG) of the EBID deposits at room temperature, i.e., the deposited structures continue to grow without further electron irradiation as long as the precursor is dosed. Due to the nature of the AG process, the corresponding deposits are also chemically well-defined. For example, the deposition of pure iron is possible via AG from the precursor Fe(CO)5.8 A second challenge that EBID faces is the unintended widening of deposits due to scattering of the high-energy electron beam in the deposit or the solid support material leading to deposit formation in areas outside the immediate impact point of the focused electron beam, the socalled electron proximity effects.9 The electron proximity effects are obviously more severe



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for higher electron doses. Therefore these unwanted effects can be suppressed by minimizing the electron dose applied to the surface during deposit formation. This can be achieved by depositing only a thin seed layer by EBID which then sustains AG of the deposit during subsequent precursor dosage until the desired nanostructure thickness is reached. However, this combined EBID + AG technique has a considerable drawback; namely, the AG occurs immediately with the EBID deposition and continues as long as EBID is performed since precursor dosage is required during the entire writing process. Therefore, one has to consider that the structures written first experience the longest AG time, which requires in turn an adjustment of the electron dose to write defined structures. The combined EBID + AG technique has led to the discovery and successive development of an alternative FEBIP technique which is referred to as electron beam induced surface activation (EBISA).10–12 In EBISA, a suitable substrate such as SiOx or TiO28,12,13 or a thin organic layer on a surface11,14 is activated by the electron beam in the absence of the precursor vapour. These reactive surface sites then initiate the decomposition of precursor molecules and consequently also the subsequent AG. The absence of precursor molecules during electron irradiation is one of the main advantages of the EBISA process as both catalytic precursor dissociation on the surface and AG occur in a parallel fashion on all activated surface areas enabling the fabrication of well-defined deposits. Secondly, for the fabrication of a solid deposit one relies on the AG only and thus electron proximity effects due to secondary or backscattered electrons are simply not-existent during the AG process. The broadening of the structure is then only a result of AG while electron proximity effects are efficiently suppressed. This enables EBISA in principle as tool for the precise fabrication of nanoscale structures and materials. One slight drawback of EBISA is that the electron doses to activate the different substrates are usually much higher than the ones needed for EBID + AG. As a result, proximity effects due to backscattered electron (note that there is no forward scattering in a deposit in EBISA) are expected to be higher. Nevertheless, even though we did ACS Paragon Plus Environment

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not put much effort in exploring the ultimate resolution in EBISA, we have been able to fabricate line patterns via EBISA + AG with line width in the order of 30 nm.11 So far, EBISA was demonstrated only in UHV with several specific precursor-substrate combinations. Fe(CO)5 and Co(CO)3NO were identified as suitable precursors for EBISA + AG yielding clean metal deposits in the case of the iron precursor and rather oxidic deposits in the case of the cobalt precursor.10,12,15 As suitable substrates, oxide surfaces,8,11,12 organic layers,11,14 and surface-anchored metal-organic frameworks (SurMOFs)10 have been identified. Remarkably, some of the substrates could be activated for both precursors, while others selectively decomposed only one of them or induced unselective decomposition on the whole substrate already at room temperature without any electron irradiation.11,12,16 While the activation mechanism on oxides could be conclusively interpreted as due to reactive oxygen vacancies locally created by electron stimulated oxygen desorption,17 the mechanism on organic and metal-organic substrates remains speculative. Therefore, to further improve the EBISA process and widen the range of applicable substrates or precursor molecules, it is of pivotal importance to further elucidate the underlying chemical mechanisms of the activation process. The SurMOF HKUST-1 was very recently successfully tested as a new type of substrate for EBID and EBISA.10 HKUST-1 is a prominent example out of a wide class of threedimensional materials that are composed of small metal centres interconnected by polyfunctional organic linkers and is particularly easy to grow on surfaces following a layerby-layer approach.18–20 Through the use of carboxylic acid or hydroxyl functionalized selfassembled monolayers which consist of molecules that bind to gold via an additional thiol functional group, HKUST-1 can be linked to gold surfaces18–20 and therefore becomes accessible as EBISA substrate.10 SurMOFs have several advantages that are anticipated to improve the spatial control of the deposit growth. First, they decouple the deposition process



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from the underlying metal surface which often supports thermal precursor dissociation even in the absence of the electron beam.21 Secondly, due to the lower density of SurMOFs, the impinging primary electrons which are subject to ballistic transport experience less scattering events than in the underlying dense substrate. This causes reduced backscattering of electrons, consequently reducing proximity effects during the irradiation step.10 Besides these favourable properties, the flexibility of using linkers with different molecular structure of functional groups does not only offer the possibility to tune the properties of the SURMOFs but also bears the potential to study the electron beam induced activation mechanism. In the present study, we have systematically varied the building blocks within surfaceanchored layers to unravel which species are crucial for the electron-induced activation of the surface in EBISA. Therefore, layers of the well-known surface-anchored MOF HKUST-110,18 are compared to layers of copper(II) oxalate grown by the same layer-by-layer approach.22,23 EBISA + AG experiments as well as EBID + AG experiments were carried out with Fe(CO)5 and Co(CO)3NO to reveal the specific activity of a given precursor-substrate combination. To obtain insight into the chemical processes occurring in the surface-anchored layers under electron irradiation, these studies on deposit shape and elemental composition performed in an UHV electron microscope were combined with additional surface science analytical techniques, namely, reflection absorption infrared spectroscopy (RAIRS) and electron stimulated desorption (ESD). These techniques monitor the degradation and chemical conversion of the organic material present in the layers

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and reveal any volatile species that

are released during electron irradiation. Based on the experimental results obtained in the present study in combination with a comprehensive literature review we are able to suggest which species most likely act as initiators of autocatalytic deposit growth following surface activation by electron irradiation.


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2. Experimental All Samples were grown on gold surfaces functionalized with carboxylic acid-terminated selfassembled monolayers (SAM) following an approach described in detail previously.22,23 Briefly, layer-by-layer deposition of the different metal-organic surface layers was carried out by alternately dipping the substrates functionalized with mercaptoundecanoic acid (Sigma-Aldrich, 95%) in ethanolic solutions of copper(II) acetate monohydrat (SigmaAldrich, >99.0%) followed by 1,3,5 benzenetricarboxylic acid (H3btc) (Acros Organics, 98%) for surface-anchored HKUST-1 and copper(II) acetate monohydrate followed by oxalic acid dihydrate (Merck, 99.5%) for copper(II) oxalate. The concentration of the solutions was 1 mM for the metal and 0.1 mM for the organic linker. The dipping times were 10 and 20 min, respectively, for the metal and linker solutions. After each dipping step, the substrates were carefully rinsed with ethanol. A total number of 50 dipping cycles was performed to generate a sufficient thickness, thus avoiding interaction with the underlying substrate in the EBISA and EBID experiments. For ESD and RAIRS experiments, samples with 10 layers were used as this thickness is sufficient to generate reliable data. To measure ESD spectra, all samples were introduced in a home build UHV chamber equipped with an electron flood-gun (FG15/40, Specs), which generates a sufficiently divergent electron beam in the range between 10 to 500 eV to grant a uniform irradiation of the samples. ESD spectra were recorded using a quadrupole mass spectrometer (RGA 300, SRS) in logging mode. This means that continuous mass scans in the range between 1050 m/z were performed in which every scan was saved. The ESD data shown herein represent an average of three mass spectra recorded at the beginning of the irradiation with an electron energy of 150 eV. To suppress signals relating to desorption from the sample holder, a background subtraction was performed using a freshly cleaned gold target as a reference. Raw data are shown for reference in the Supporting Information (SI, Figure S1).



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To monitor the layer growth as well as the electron induced decomposition reflection absorption infrared (RAIR) spectra between 4000 and 700 cm−1 were recorded using an evacuated FTIR spectrometer (IFS 66v/S, Bruker Optics GmbH) by accumulating 400 scans. The spectrometer was equipped with a grazing incidence reflection unit and a liquid nitrogencooled MCT detector with the sensitivity ranging down to 750 cm−1. The resolution was set to 4 cm-1 and the aperture to 2 mm. The chamber pressure was 5 mbar. The system was purged with N2 to eliminate water vapor and carbon dioxide. Background spectra were recorded on a mercaptoundecanoic acid SAM. The lithographic experiments were performed in a commercial UHV system (Multiscanlab, Omicron Nanotechnology, Germany) with a base pressure of p < 2×10−10 mbar. The main component of the analysis chamber is a UHV-compatible electron column (Leo Gemini) for SEM with a nominal resolution better than 3 nm. In combination with a hemispherical electron energy analyzer, local AES and scanning Auger microscopy with a nominal resolution better than 10 nm is possible. Fe(CO)5 was purchased from ACROS Organics, Co(CO)3NO from abcr GmbH&Co. KG. The quality of the precursor gas was analyzed with a quadrupole mass spectrometer in a dedicated gas analysis chamber (base pressure

Electron Beam Induced Surface Activation of the Metal- Organic

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