Thioether-Functionalized Quinone-Based Resorcin[4]arene Cavitands: Electroswitchable Molecular Actuators

The utility of molecular actuators in nanoelectronics requires activation of mechanical motion by electric charge at the interface with conductive surfaces. We functionalized redox-active resorcin[4]arene-quinone cavitands with thioethers as surface-anchoring groups at the lower rim and investigated their propensity to act as electroswitchable actuators that can adopt two different conformations in response to changes in applied potential. Molecular design was assessed by DFT calculations and X-ray analysis. Electronic properties were experimentally studied in solution and thin films electrochemically, as well as by X-ray photoelectron spectroscopy on gold substrates. The redox interconversion between the oxidized (quinone, Q) and the reduced (semiquinone, SQ) state was monitored by UV-Vis-NIR spectroelectrochemistry and EPR spectroscopy. Reduction to the SQ state induces a conformational change, providing the basis for potential voltage-controlled molecular actuating devices.

Stability and physical properties of alkylsilane and alkylphosphonate self-assembled monolayer (SAM) films on metal oxide substrates characterized at the micro- and nanoscale

Enamul Hoque

Self-assembled monolayer (SAM) films have attracted immense attention for both fundamental and applied research. A SAM is composed of a large number of molecules with a head group that chemisorbs onto a substrate, a tail group that interacts with the outer surface of the film, and a spacer (backbone) chain group that connects the head and tail groups resulting in a coating. Interactions between spacer groups of different molecules, such as van der Waals forces and/or hydrogen bonding, hasten SAM film formation and contribute to its stability. In this dissertation, SAM and thin films have been formed onto copper and aluminum oxide surfaces by reaction with 1H,1H,2H,2H-perfluorodecyldimethylchlorosilane (PFMS), 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFTS), 1H,1H,2H,2H-perfluorodecylphosphonic acid (PFDP), octylphosphonic acid (OP), decylphosphonic acid (DP), and octadecylphosphonic acid (ODP). The properties and stability of the films were investigated employing complementary surface analysis techniques: X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), friction force microscopy (FFM), a derivative of AFM, contact angle measurements (CAMs), and Fourier transform infrared reflection/absorption spectroscopy (FT-IRRAS). The perfluoroalkylsilane SAM on Cu is found to be extremely hydrophobic typically having sessile drop static contact angles of more than 130° for pure water and a surface energy of 14 mJ/m2 (mN/m). FFM showed a significant reduction in the adhesive force and friction coefficient of PFMS modified Cu (PFMS/Cu) compared to unmodified Cu. Treatment by exposure to harsh conditions showed that PFMS/Cu SAM can withstand boiling nitric acid (pH=1.8), boiling water, and warm sodium hydroxide (pH=12, 60 °C) solutions for at least 30 minutes. Furthermore, no SAM degradation was observed when PFMS/Cu was exposed to warm nitric acid solution for up to 70 min at 60 °C or 50 min at 80 °C. XPS and FT-IRRAS data reveal a coordination of the PFMS silicon (Si) atom with a cuprate (CuO) molecule present on the oxidized copper substrate. The data give good evidence that the stability of the SAM film on the PFMS modified oxidized Cu surface is largely due to the formation of a siloxy-copper (-Si-O-Cu-) bond via a condensation reaction between silanol (-Si-OH) and copper hydroxide (CuOH). For a PFTS modified Cu surface (PFTS/Cu), the sessile drop static contact angle of pure water has been measured to be more than 125° and the surface energy to be typically less than 16 mJ/m2. Stability tests show that the PFTS/Cu film can survive in boiling pure water for one hour, boiling nitric acid (pH 1.5 or 1.8) for 30 minutes, sodium hydroxide solution (pH 12, 70 °C) for 30 minutes, and autoclave conditions (steam at 134 °C and 3 atmospheres) for 15 minutes. The more commonly used self-assembled monolayer (SAM) modifications of Cu surfaces, e.g. thiol compounds, are significantly less stable than PFTS/Cu. Extremely hydrophobic (low surface energy) and stable PFMS/Cu SAMs and PFTS/Cu films could be useful as corrosion inhibitors in micro/nanoelectronic devices and/or as promoters for anti-wetting, low adhesion surfaces or drop-wise condensation on heat exchange surfaces. XPS analysis confirmed the presence of perfluorinated and non-perfluorinated alkylphosphonate molecules on the PFDP, DP, and ODP SAMs deposited at the aluminum oxide coated silicon (Al/Si) surfaces. The sessile drop static contact angle of pure water on PFDP SAMs was typically more than 130° and on DP and ODP typically more than 125° indicating that the phosphonic acid SAMs reacted with Al samples were very hydrophobic. The surface roughness for PFDP/Al, DP/Al, ODP/Al, and bare Al was approximately 35 nm, as determined by AFM. The surface energy for PFDP/Al was determined to be approximately 11 mJ/m2 by the Zisman plot method compared to 21 mJ/m2 and 20 mJ/m2 for DP/Al and ODP/Al, respectively. PFDP/Al gave the lowest adhesion and friction force while bare Al gave the highest. The adhesion and friction forces for ODP/Al and DP/Al SAMs were in between. ODP, DP, and OP SAMs have been studied in detail on relatively flat aluminum oxide surfaces. The rms surface roughness for ODP/Al, DP/Al, OP/Al, and bare Al was less than 15 nm, as determined by AFM. The sessile drop static contact angle of pure water on ODP/Al and DP/Al was typically more than 115° and on OP/Al typically less than 105°. The surface energy for ODP/Al and DP/Al was determined to be approximately 21 mJ/m2 and 22 mJ/m2, respectively, compared to 26 mJ/m2 for OP/Al. ODP/Al and OP/Al were studied by FFM to better understand their micro-/nano-tribological properties. ODP/Al gave the lowest coefficient of friction values while bare Al gave the highest. The adhesion forces for ODP/Al and OP/Al were comparable. The chemical stability of ODP/Al, PFDP/Al, DP/Al, OP/Al, and PFMS/Al SAMs has been inspected by exposure to warm nitric acid (pH 1.8, 30 min, 60-95 °C). The XPS data and stability against harsh chemical conditions indicate that a type of bond forms between a phosphonic acid (PA) or silane molecule and the oxidized Al surface. Stability tests using warm nitric acid (pH 1.8, 30 min, 60-95 °C) show ODP/Al SAMs to be most stable followed by PFDP/Al, DP/Al, PFMS/Al, and OP/Al SAMs. For PFTS/Al, stability tests demonstrate that modified aluminum is able to survive exposure to warm nitric acid (pH = 1.8, 60 °C, 30 min) indicating some degree of robustness. Hydrophobic, low adhesion, and robust aluminum surfaces have useful applications for micro/nano-electromechanical systems (MEMS/NEMS), such as digital micro-mirror devices (DMDs). These studies are expected to aid in the design and selection of proper lubricants for MEMS/NEMS.

EPFL2007

Scanning Tunneling Luminescence of Pentacene Nanocrystals

Alexander Kabakchiev

Organic semiconductors are promising materials for future electronic and electroluminescence applications. A detailed understanding of organic layers and nano-sized crystals down to single molecules can address fundamental questions of contacting organic semiconductors at the nanometer limit and obtaining luminescence from them. In this thesis, electroluminescence spectra from pentacene, a policyclic hydrocarbon (acene), are discussed. The luminescence is induced by the current from the tip of a scanning tunneling microscope (STM). Pentacene is an organic semiconductor which gained a lot of attention in technology because of its electronic properties suitable for applications in thin film devices. Moreover, recent fundamental studies employed pentacene as a standard to demonstrate sub-molecular resolution by scanning probe techniques. This work reports the first observation of light emission from nanometer-sized pentacene crystals grown on an ultrathin insulating layer on noble metal surfaces. Different STM-techniques are combined to characterize the individual systems studied with respect to topography, crystal structure, electronic properties and work function changes. The initial step of the project was the implementation of an optical system for light collection from the tunnel-junction of a low-temperature STM. In the set-up, a novel approach based on the use of in situ adjustable lenses has been realized. Three lenses placed in the vicinity of the tip-apex allow light collection into three independent channels which can be used for versatile optical analysis. An important part in the characterization of the organic system was to clarify the interaction between adsorbed molecules and substrates. We investigated individual pentacene molecules on different ultrathin insulator-metal systems. With the STM tip in tunnel contact, the molecules are situated in a double barrier junction formed by the insulating layer on one side and the vacuum gap on the other side. The metal surface and the STM-tip form the electrodes. The electronic properties of pentacene in this configuration have been characterized. Insulator-metal-systems, which provide a good electronic decoupling for pentacene from the metal, have been chosen as substrates for the growth of pentacene nanocrystals. Using the sub-molecular resolution of the STM we resolved the structure of the top-layer of the pentacene nanocrystals and found that the crystal phase agrees with the pentacene bulk structure. Moreover, the comparison of charge injection barriers between individual molecules and nanocrystals of pentacene indicates a significant change of electronic properties after the formation of ordered structure from the individual building blocks. Optical spectroscopy of the emitted light reveals an excitonic emission from the nanocrystals, which is in very good agreement with photoemission spectra of macroscopic crystals. Although a highly localized current-injection by the STM tip is used for excitation, it can be concluded that the source of light emission is delocalized. In contrast to luminescence measurements reported for other organic materials in STM, the excitation is not localized on the individual molecule, into which a charge is injected by the STM-tip. Our study indicates the importance of inter-molecular coupling leading to a high mobility of the excited state which has to be considered in the framework of STM-induced luminescence. A further result is the observation of the Stark shift of pentacene luminescence, that has so far not been measured at large electric fields of the order of 1V/nm. The evaluation of the Stark shift provides information on dipole and polarizability change during the decay of an exciton. The measured large dipole change of the lowest singlet exciton in pentacene indicates a mixing of the Frenkel exciton (FE) with a small contribution of charge-transfer (CT). Finally, we developed a set-up for photon correlation measurements in the STM. The properties of photon statistics is a means which can unambiguously prove that the luminescence is due to a single-photon source, i.e. a light source which emits no more than one photon at a time. A typical example of such a source is the emission from a single molecule. We performed photon correlations measurements of the luminescence from pentacene and C60 nanocrystals. While the excitonic bulk emission of pentacene is not expected to represent a single photon emitter, we also did not observe anticorrelations in the case of C60 indicating the importance of the local environment in the STM-junction, e.g. the short distance to the metal electrodes.

EPFL2010

Formation Mechanisms and Structural Properties of Aluminium-Chromium Based Oxynitride Thin Films Synthesized by Physical Vapor Deposition

Hossein Najafi

Oxynitride thin films, a novel class of mixed anionic solid structures, have gained an intense interest during recent years related to the unique interplay of oxygen (O) and nitrogen (N) with metals in the material structure. This matter can result in specific physical and structural properties. Accordingly, by varying the anionic composition (O/(O+N)), it is possible to tune the properties of oxynitrides, and particularly to enhance mechanical strength and phase stability. Consequently, the understanding of the role of O and N in such structures is of great interest. Taking this into account, we studied the formation mechanisms and properties of AlCr-based oxynitride films synthesized by the reactive magnetron sputtering (650◦C) and cathodic arc evaporation (550◦C) with an emphasis for application in hard coat- ings. Our results revealed that the variation of the anionic composition (O/(O+N)) disclosed the existence of three distinct growth regimes including: (i) a nitride regime O/(O+N)≤ 0.6, (ii) a transition regime 0.6 < O/(O+N) < 0.97, and (iii) an oxide regime O/(O+N)≥ 0.97, which were correlated to the structural features, chemical environ- ment, physical and mechanical properties of the films within each regime. For the oxygen fraction up to 60%, transmission electron microscopy (TEM) and x-ray diffrac- tion (XRD) alongside indicate the development of films in the nitride-type structure with a continues anionic substitutions, i.e. solid solution of AlCrOx N1−x . With fur- ther increase of the oxygen content, the structure of the layers is that of a poorly crystallized phase, and a near-amorphous film was favored in accordance with the observed low nanohardness, i.e. transition regime. Finally, occurring at the highest oxygen content, oxide regime (O/(O+N)≥ 0.97) was assigned to the films with oxide- type structure; however, with some probable oxygen substitution by nitrogen. The composition/structure variation was consistent with the chemical environment of the involved atoms, evaluated by x-ray photoelectron spectroscopy (XPS), where the electronic properties of the films exhibited substantial changes from regime to regime. Moreover, the film properties observed within each regime were also strongly in- fluenced by the cationic composition and deposition conditions. Al-rich layers (Cr/Al ≤ 0.1) were found to crystallize in the hexagonal wurtzite (B4) structure in the nitride regime (O/(O+N) ≤ 0.55) and transformed to a fully amorphous network and cubic γ-(Al,Cr)2O3 in the transition and oxide regimes, respectively. Conversely, Cr-rich films (Cr/Al≃1) were crystallized in the cubic (B1) structure within the nitride regime and preserved their structure in the transition regime despite the large pro- portion of oxygen. Such coatings in the oxide regime consisted of a solid solution of α-(Al,Cr)2(O0.97,N0.03)3 with corundum lattice and favorable protective proper- ties. Furthermore, it was observed that the structure of the films deposited at lower temperatures (400◦C) remains in the transition regime, and the oxide structure is not developed. The presence of silicon (Si, 3-4 at.%) and yttrium (Y≤ 1 at.%) in the oxynitride films was found to change the thresholds and structural features within the growth regimes. Oxynitride layers exhibited an enhanced phase stability (i.e. cubic-B1) by the incorporation of both Si and Y. As a result, the nitride regime was extended in the range of O/(O+N)≤ 0.8. However, on the other hand, Si and Y prohibit the formation of corundum phase in the oxide regime suggesting retarded diffusional processes with the presence of such alloying elements. Considering the preparation conditions of the oxide coatings, we found a previously unobserved formation of cubic crystalline fcc-(Al,Cr)2O3 and its transition to the well-known corundum phase (α- (Al,Cr)2O3) at higher film thickness beyond 2 μm by employing an extra-high substrate bias during arc evaporation. The TEM, XRD, and XPS results alongside indicated that the cubic (B1) layer acquires the properties of a metastable monoxide (M1−x O), which is kinetically stabilized by the occurrence of vacancies. It was proposed that there is a thickness-dependent gradient in point defect density, and its modification leads to the destabilized fcc-(Al,Cr)2O3 and a cubic-to-corundum transformation occurs. Favorable phase stability and oxidation resistance together with the low wear rates and friction behavior observed for the oxynitride films underline their potential for hard coating of tools used under severe dry cutting conditions.

EPFL2012