Article pubs.acs.org/Langmuir
Organic Monolayers from 1‑Alkynes Covalently Attached to Chromium Nitride: Alkyl and Fluoroalkyl Termination Sidharam P. Pujari,† Luc Scheres,‡ Barend van Lagen,† and Han Zuilhof*,†,§ †
Laboratory of Organic Chemistry, Wageningen University, Dreijenplein 8, 6703 HB Wageningen, The Netherlands Surfix B.V., Dreijenplein 8, 6703 HB Wageningen, The Netherlands § Department of Chemical and Materials Engineering, King Abdulaziz University, Jeddah, Saudi Arabia ‡
S Supporting Information *
ABSTRACT: Strategies to modify chromium nitride (CrN) surfaces are important because of the increasing applications of these materials in various areas such as hybrid electronics, medical implants, diffusion barrier layers, corrosion inhibition, and wettability control. The present work presents the first surface immobilization of alkyl and perfluoro-alkyl (from C6 to C18) chains onto CrN substrates using appropriately functionalized 1-alkynes, yielding covalently bound, high-density organic monolayers with excellent hydrophobic properties and a high degree of short-range order. The obtained monolayers were characterized in detail by water contact angle, X-ray photoelectron spectroscopy (XPS), ellipsometry, and infrared reflection absorption spectroscopy (IRRAS).
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conditions or could be fine-tuned to minimize the response by the immune system upon placement of a medical implant in the body. Such applications would require a precisely tuned modification of the surface, such as that obtainable via covalently attached organic monolayers or polymers. Chemical modifications via the covalent attachment of organic monolayers are employed via a wide variety of approaches, depending on the nature of the substrate. This includes, for example, the adsorption of alkanethiols, dialkyl disulfides, and dialkyl sulfides on gold10 and GaAs(001),11 fatty acids12 and alkenes3 on indium tin oxide (ITO), alkenes on oxide-free Cu,13 alkanephosphonic acids on titanium,14 and organosilanes on silicon surfaces used for the development of MEMS and NEMS.15 However, although interesting for a wide variety of properties ranging from electronic modification such as band bending and surface dipoles to mechanical changes including friction and adhesion control, these monolayers are not always very stable. This need for a high stability of these molecular coatings in real-life applications motivated researchers to use other interesting functional groups such as alkynes on SiC and SiN surfaces, which show a very high stability (e.g., stable in refluxing acid at pH 99%, PMA) were used as received. Monolayer Formation. A chromium nitride (CrN) epilayer on a Si(100) substrate was diced into 10 mm ×10 mm wafers with a diamond-tipped pen. The CrN surface was first cleaned by rinsing several times with dichloromethane and sonication for 10 min in acetone. Subsequently, the samples were further activated using air plasma (PDC-002 plasma cleaner, Harrick Scientific Products, Inc. Ossining, NY) for 10 min (0.3 SCFH air flow, 29.6 W power, 300 mTorr pressure) to remove adventitious organic contamination. After being activated, the samples were blown dry with a stream of argon. These freshly plasma-activated and dried surfaces were then quickly transferred to a screw-capped bottle under an argon atmosphere, which was charged with 1 mL of alkynes at 100 °C for 16 h or for the time and at the temperature indicated in the text. After reaction, the samples were removed from the flask, rinsed extensively with DCM, and sonicated for 5 min in acetone to remove physisorbed reagents. Samples were stored in a glovebox between measurements, rinsed with fresh DCM, and blown dry with argon immediately before characterization. 2-Hydroxyhexadecanoic acid (2HHDA)-modified surfaces were prepared using an activation process similar to that described elsewhere.29 In this case, after plasma activation the samples were rinsed in ethanol and immersed in a 1 mM solution of 2hydroxyhexadecanoic acid in ethanol at 65 °C for 16 h. This same procedure was used to prepare palmitic acid (PMA)-derived monolayers. Surface Characterization. The static and advancing water contact angles of the bare and modified CrN surfaces were measured using the sessile drop method on a DSA100 optical contact angle meter (Krüss instrument). The thickness of the alkynes grafted onto the CrN substrate was determined using a Sentech Instruments (type SE-400) automated ellipsometer. The optical constants of the substrate were determined with a piece of freshly plasma-oxidized CrN with a refractive index of ns = 2.73 and an imaginary refractive index of ks = 2.48. Each reported value of the layer thickness is the average of a minimum of eight measurements taken at different locations on the substrate with an error ±0.3 nm. The elemental composition of the modified CrN surfaces was determined by X-ray photoelectron spectroscopy (XPS) using a JPS-9200 photoelectron spectrometer (JEOL, Japan). All spectra were corrected with a slight linear background before fitting. All XPS spectra were evaluated using Casa XPS software (version 2.3.15), and the C 1s hydrocarbon CH2 peak was calibrated at a binding energy (BE) of 285.0 eV. Calculated atomic percentages were normalized by the corresponding atomic sensitivity factors [C 1s (1.00), Cr 2p (10.6), N 1s (1.80), O 1s (2.93), F 1s (4.43), S 2p (1.68), and Cl2p (2.29); http://www.casaxps.com/] for the X-rays incident at 80° from the analyzer, as shown in Table 1 (vide infra). Infrared reflection absorption spectroscopy (IRRAS) spectra were measured using a Bruker TENSOR 27 equipped with a liquid-nitrogen-cooled MCT (mercury, cadmium, telluride) detector using a Harrick Auto SeaguII with a variable angle (10−85°) attachment. For our experiments, the angle of p-polarized light
Table 1. Static Water Contact Angle and XPS-Based Elemental Composition (in Atom %) of Chromium Nitride after Various Surface Treatments surface treatments untreated CrN air plasma HF (2.5%)a piranhaa 1 M HCl a
C 1s (285.0 eV)
Cr 2p (575.0 eV)
N 1s (397 eV)
O 1s (531 eV)
37.2
26.8
17.2
18.7
9.2 8.7 8.2 10.0
33.3 33.3 30.3 32.5
16.4 16.1 15.4 15.1
41.1 36.2 42.6 40.3
other (F 1s (684 eV) S 2p (168 eV))
contact angle (deg) 106
F 1s, 5.6 S 2p, 3.3 Cl 2p, 2.1
