Azobenzene Monolayer Research Articles

Photo-Programmable Processes in Bithiophene–Azobenzene Monolayers on Gold Probed via Simulations

In this study, we investigate the structural changes, electronic properties, and charge redistribution within azo-bithiophene (Azo-BT)-chemisorbed monolayers under different light stimuli using the density functional theory and molecular dynamics simulations. We consider two types of switches, Azo-BT and BT-Azo, with different arrangements of the Azo and BT blocks counting from the anchor thiol group. The chemisorbed monolayers of pure cis- and trans-isomers with a surface concentration of approximately 2.7 molecules per nm2 are modeled on a gold surface using the classical all-atom molecular dynamics. Our results reveal a significant shrinkage of the BT-Azo layer under UV illumination, whereas the thicknesses of the Azo-BT layer remain comparable for both isomers. This difference in behavior is attributed to the ordering of the trans-molecules in the layers, which is more pronounced for Azo-BT, leading to a narrow distribution of the inclination angle to the gold surface. Conversely, both layers consisting of cis-switches exhibit disorder, resulting in similar brush heights. To study charge transfer within the immobilized layers, we analyze each snapshot of the layer and calculate the mean charge transfer integrals using Nelsen’s algorithm for a number of interacting neighboring molecules. Combining these integrals with reorganization energies defined for the isolated molecules, we evaluate the charge transfer rates and mobilities for electron and hole hopping within the layers at room temperature based on Marcus’ theory. This research offers new perspectives for the innovative design of electrode surface modifications and provides insights into controlling charge transfer within immobilized layers using light triggers. Additionally, we identify molecular properties that are enhanced via specific molecular design, which contributes to the development of more efficient molecular switches for various electronic applications.

Anchoring transition induced by trans-cis isomerization in photosensitive substrate.

A molecular model describing the effective anchoring energy of a liquid crystal (LC) system, composed of 4-n-pentyl-4^{'}-cyanobiphenyl (5CB) molecules deposited on a photosensitive azobenzene layer consisting of 6Az10PVA molecules, is proposed. This model takes into account the interaction between the surface polarization in the LC phase and the surface electric field, arising from the surface charge density. Within the framework of this molecular model, the mechanism responsible for the anchoring transition in the LC phase from homeotropic to planar alignment and vice versa, caused by trans-cis and cis-trans conformational changes in the monolayer 6Az10PVA after laser radiation, is described. It is shown that using experimental data for the voltage across the 6Az10PVA+5CB film, obtained by the surface potential technique, the charge separation during the conformational changing, caused by the laser irradiation, may lead to an anchoring transition induced by trans-cis-trans isomerization in the photosensitive azobenzene monolayer. The calculated values of the isothermal surface pressure diagram π-A showed that the surface area A per LC molecule is noticeably higher for the cis configuration, which reflects a less efficient packaging than in the trans state.

Modulating the Optoelectronic Properties of MoS2 by Highly Oriented Dipole-Generating Monolayers.

The noncovalent functionalization of two-dimensional materials (2DMs) with bespoke organic molecules is of central importance for future nanoscale electronic devices. Of particular interest is the incorporation of molecular functionalities that can modulate the physicochemical properties of the 2DMs via noninvasive external stimuli. In this study, we present the reversible modulation of the photoluminescence, spectroscopic properties (Raman), and charge transport characteristics of molybdenum disulfide (MoS2)-based devices via photoisomerization of a self-assembled monolayer of azobenzene-modified triazatriangulene molecules. The observed (opto)electronic modulations are explained by the n-type doping of the MoS2 lattice induced by the photoisomerization of the highly ordered azobenzene monolayer. This novel behavior could have profound effects on future composite 2DM-based (opto)electronics.

Microfluidics of liquid crystals induced by laser radiation.

Several scenarios for the formation of hydrodynamic flows in microsized hybrid aligned nematic (HAN) channels, based on the appropriate nonlinear extension of the classical Ericksen-Leslie theory, supplemented by thermomechanical correction of the shear stress and Rayleigh dissipation function, as well as taking into account the entropy balance equation, are analyzed. Detailed numerical simulations were performed to elucidate the role of the heat flux q caused by laser radiation focused on the lower boundary of the equally warmed up the HAN channel containing a monolayer of azobenzene with the possibility of a trans-cis and cis-trans conformational changes in formation of the vortex flow v. It is shown that a thermally excited vortex flow is maintained with motion in a positive sense (clockwise) in the vicinity of the orientation defect at the lower boundary of the HAN channel caused by the trans-cis and cis-trans conformational changes. In the case of the same HAN channel, but without the azobenzene monolayer at the lower boundary, the heat flux q can also produce the vortical flow in the vicinity of the laser spot at the lower boundary, directed in a negative sense (counterclockwise). At that, the second vortex is characterized by a much slower speed than the vortical flow in the first case.

Vibrational sum-frequency generation study of molecular structure, sterical constraints and nonlinear optical switching contrast of mixed alkyl-azobenzene self-assembled monolayers

AbstractSelf-assembled monolayers (SAMs) of azobenzene (AB) functionalized alkyl thiols on gold diluted with simple alkyl thiols provide a straightforward way to photochromic surfaces with high and tunable photoswitching efficiency.Trans-cisisomerization of the AB molecule changes the physical properties of the surface, including the nonlinear optical (NLO) response. Vibrational sum-frequency generation (VSFG) spectroscopy as a nonlinear type of laser spectroscopy offers surface- and orientation-sensitive insight into the molecular structure of mixed SAMs. In this study, VSFG as well as ultraviolet-visible (UV/Vis) spectroscopy has been employed to investigate the morphology, molecular structure, and NLO response of mixed SAMs with systematically varied surface composition. Methylazobenzene (MeAB) has been used as the molecular switch with the methyl substituent serving as orientational VSFG marker. Both short-chain and long-chain alkyl thiol co-ligands have been used to gain insight into the interplay between SAM structure and sterical constraints that are known to limit the free switching volume. Underlining the dominating role of sterical effects for controlling photochromic properties, a strong inhibition of the photoswitching efficiency and NLO response has been observed for the SAMs with an alkyl thiol co-ligand long enough to spatially extend into the layer of the MeAB chromophore. Overall, with <12% signal change, the relative NLO switching contrasts remained low in all cases. VSFG spectral trends clearly revealed that the presumably higher photoswitching efficiency upon dilution with the co-ligand is counteracted by a loss of structural order of the chromophore.

All-optical reversible control of integrated resonant cavity by a self-assembled azobenzene monolayer.

The next frontier in photonics will rely on the synergistic combination of disparate material systems. One unique organic molecule is azobenzene. This molecule can reversibly change conformations when optically excited in the blue (trans-to-cis) or mid-IR (cis-to-trans). Here, we form an oriented monolayer of azobenzene-containing 4-(4-diethylaminophenylazo)pyridine (Aazo) on SiO2 optical resonators. Due to the uniformity of the Aazo layers, quality factors over 106 are achieved. To control the photo-response, the density of Aazo groups is tuned by integrating methyl spacer molecules. Using a pair of lasers, the molecule is reversibly flipped between molecular conformations, inducing a refractive index change which results in a resonant wavelength shift. The magnitude of the shift scales with the relative surface density of Aazo. To investigate reproducibility and stability of the organic monolayer, three switching cycles are demonstrated, and the performance is consistent even after a device is stored in air for 6 months.

Mechanically Induced Switching of Molecular Layers.

Within the field of switchable surfaces, azobenzenes are an extensively studied group of molecules, known for reversibly changing conformation upon illumination with light of different wavelengths. Relying on the ability of the molecules to change properties and structure as a response to external stimuli, they have been incorporated in various devices, such as molecular switches and motors. In contrast to the well-documented switching by light irradiation, we report the discovery of mechanically triggered switching of self-assembled azobenzene monolayers, resulting in changes of surface wettability, adhesion, and friction. This mechanically induced cis-trans isomerization is triggered either locally and selectively by AFM or macroscopically by particle impact. The process is optically reversible, enabling consecutive switching cycles. Collective switching behavior was also observed, propagating from the original point of impact in a domino-like manner. Finally, local force application facilitated nondestructive and erasable nanopatterning, the cis-trans nanolithography.

Nanoscale Raman Imaging with Nanogold-Topped AFM Probes Fabricated by Area-Selective Electroless Deposition

A single gold nanostructure is successfully deposited at the tip apex of a silicon probe for atomic force microscopes (AFM) in an electroless and area-selective manner. The tip apex of a commercially available silicon AFM probe is irradiated with a focused ion beam (FIB) to remove the native oxide layer, and then the probe was exposed to chloroauric acid (HAuCl4) aqueous solution containing sodium chloride (NaCl). A single gold nanostructure selectively grows at the FIB-irradiated apex as a consequence of electron transfer from silicon to the solution through the FIB-irradiated silicon surface. This electron transfer is driven by the difference in the electrochemical potential of electrons. NaCl, added to a pure HAuCl4 aqueous solution, improves the area-selectivity of gold growth and decreases the size of gold, resulting in desired geometry of gold as the tip apex for tip-enhanced Raman spectroscopy (TERS) in terms of the spatial resolution and the enhancement of the electric field. Our probes provide TERS imaging of a carbon nanotube with a spatial resolution of 10 nm, and enhancement factors up to 2.2 × 105 for an azobenzene thiol self-assembled monolayer in the gap-mode configuration with a side-illumination optical setup.

Light propagation in a photonic crystal fiber infiltrated with mesogenic azobenzene dyes

In this paper, light propagation in an isotropic photonic crystal fiber as well in a silica-glass microcapillary infiltrated with a mesogenic azobenzene dye has been investigated. It appeared that light spectrum guided inside the photonic crystal fiber infiltrated with the investigated azobenzene dye depends on the illuminating wavelength of the absorption band and on linear polarization. Also, alignment of the mesogenic azobenzene dye molecules inside silica glass microcapillaries and photonic crystal fibers has been investigated. Results obtained may lead to a new design of optically tunable photonic devices. Full Text: PDF ReferencesP. Russell. St. J. "Photonic-Crystal Fibers", J. Lightwave Technol. 24, 4729 (2006). CrossRef T. Larsen, A. Bjarklev, D. Hermann, J. Broeng, "Optical devices based on liquid crystal photonic bandgap fibres", Opt. Exp. 11, 2589 (2003). CrossRef D. C. Zografopoulos, A. Asquini, E. E. Kriezis, A. d'Alessandro, R. Beccherelli, "Guided-wave liquid-crystal photonics", Lab Chip, 12, 3598 (2012). CrossRef F. Du, Y-Q. Lu, S-T. Wu, "Electrically tunable liquid-crystal photonic crystal fiber", Appl. Phys. Lett 85, 2181 (2004) CrossRef D. C. Zografopoulos, E. E. Kriezis, "Tunable Polarization Properties of Hybrid-Guiding Liquid-Crystal Photonic Crystal Fibers", J. Lightwave Technol. 27 (6), 773 (2009) CrossRef S. Ertman, M. Tefelska, M. Chychłowski, A. Rodriquez, D. Pysz, R. Buczyński, E. Nowinowski-Kruszelnicki, R. Dąbrowski, T. R. Woliński. "Index Guiding Photonic Liquid Crystal Fibers for Practical Applications", J. Lightwave Technol. 30, 1208 (2012). CrossRef D. Noordegraaf, L. Scolari, J. Laegsgaard, L. Rindorf, T. T. Alkeskjold, "Electrically and mechanically induced long period gratings in liquid crystal photonic bandgap fibers", Opt. Expr. 15, 7901 (2007) CrossRef M. M. Tefelska, M. S. Chychlowski, T. R. Wolinski, R. Dabrowski, W. Rejmer, E. Nowinowski-Kruszelnicki, P. Mergo, "Photonic Band Gap Fibers with Novel Chiral Nematic and Low-Birefringence Nematic Liquid Crystals", Mol. Cryst. Liq. Cryst. 558(1), 184 (2012). CrossRef S. Mathews, Y. Semenova, G. Farrell, "Electronic tunability of ferroelectric liquid crystal infiltrated photonic crystal fibre", Electronics Letters, 45(12), 617 (2009). CrossRef V. Chigrinov, H-S Kwok, H. Takada, H. Takatsu, "Photo-aligning by azo-dyes: Physics and applications", Liquid Crystals Today, 14:4, 1-15, (2005) CrossRef A. Siarkowska, M. Jóźwik, S. Ertman, T.R. Woliński, V.G. Chigrinov, "Photo-alignment of liquid crystals in micro capillaries with point-by-point irradiation", Opto-Electon. Rev. 22, 178 (2014); CrossRef D. Budaszewski, A. K. Srivastava, A. M. W. Tam, T. R. Woliński, V. G. Chigrinov, H-S. Kwok, "Photo-aligned ferroelectric liquid crystals in microchannels", Opt. Lett. 39, 16 (2014) CrossRef J-H Liou, T-H. Chang, T. Lin, Ch-P. Yu, "Reversible photo-induced long-period fiber gratings in photonic liquid crystal fibers", Opt. Expr. 19, (7), 6756, (2011) CrossRef T. T. Alkeskjold, J. Laegsgaard, A. Bjarklev, D. S. Hermann, J. Broeng, J. Li, S-T. Wu, "All-optical modulation in dye-doped nematic liquid crystal photonic bandgap fibers", Opt. Exp, 12 (24), 5857 (2004) CrossRef K. Ichimura, Y. Suzuki, T. Seki, A. Hosoki, K. Aoki, "Reversible change in alignment mode of nematic liquid crystals regulated photochemically by command surfaces modified with an azobenzene monolayer", Langmuir, 4, 1214 (1988) CrossRef http://www.beamco.com/Azobenzene-liquid-crystals DirectLink K. A. Rutkowska, K. Orzechowski, M. Sierakowski, "Wedge-cell technique as a simple and effective method for chromatic dispersion determination of liquid crystals", Phot. Lett, Poland, 8(2), 51 (2016). CrossRef L. Deng, H.-K. Liu, "Nonlinear optical limiting of the azo dye methyl-red doped nematic liquid crystalline films", Opt. Eng. 42, 2936-2941 (2003). CrossRef J. Si, J. Qiu, J. Guo, M. Wang, K. Hirao, "Photoinduced birefringence of azodye-doped materials by a femtosecond laser", Appl. Opt., 42, 7170-7173 (2008). CrossRef

Graphene-modulated photo-absorption in adsorbed azobenzene monolayers.

The impact of graphene on the photo-absorption properties of trans- and cis-azobenzene monolayers is studied in the framework of density-functional theory and many-body perturbation theory. We find that, despite the weak hybridization between the electronic bands of graphene and those of the azobenzene monolayers, graphene remarkably modulates the absorption spectra of the adsorbates. The excitation energies are affected via two counteracting mechanisms: substrate polarization reduces the band-gap of azobenzene, and enhanced dielectric screening weakens the attractive interaction between electrons and holes. The competition between these two effects gives rise to an overall blueshift of peaks stemming from intramolecular excitations, and a redshift of peaks from intermolecular ones. Even more interesting is that excitations corresponding to intermolecular electron-hole pairs, which are dark in the isolated monolayers, are activated by the graphene substrate. Our results demonstrate that the photoisomerization process of weakly adsorbed azobenzene undergoes notable changes on a carbon-based substrate.

Tuning the collective switching behavior of azobenzene/Au hybrid materials: flexible versus rigid azobenzene backbones and Au(111) surfaces versus curved Au nanoparticles.

The combination of photo-responsive azobenzene (AB) and biocompatible Au nanomaterials possesses potential applications in diverse fields such as biosensing and thermotherapy. To explore the influence of azobenzene moieties and Au substrates on the collective switching behavior, two different azobenzene derivatives (rigid biphenyl-controlled versus flexible alkoxyl chain-linked) and three different Au substrates (a planar Au(111) surface, curved Au102(SR)44 and Au25(SR)18 clusters) were chosen to form six Au@AB combinations. A reactive molecular dynamics (RMD) model considering both the torsion and inversion path was implemented to simulate the collective photo-induced cis-to-trans switching process of AB monolayers on Au substrates. The major driving force for isomerization is demonstrated to be the torsion of the C-N[double bond, length as m-dash]N-C dihedral angle, in addition to the minor contribution from an inversion pathway. The isomerization process can be divided into the preliminary conformation switching stage and the later relaxation stage, in which a gradual self-organization is observed for 40 ps. The Au substrate affects the packing structure of the AB monolayer, while the choice of different kinds of ABs tunes the intermolecular interaction in the monolayer. Flexible alkoxyl-linked F-AB may achieve much faster conversion on Au clusters than on the surface. For rigid biphenyl-based R-AB anchored on Au nanoparticles (AuNPs), a competitive torsion between the biphenyl and C-N[double bond, length as m-dash]N-C dihedral may delay the C-N[double bond, length as m-dash]N-C dihedral torsion and the following isomerization process. After the R-AB molecules were anchored on the Au(111) surface, the strong π-π stacking between biphenyl units accelerates the collective isomerization process. A curvature-dependent effect is observed for R-AB SAMs on different-sized substrates. The cooperation between functional AB monolayers and the Au substrate determines the collective switching behavior of Au@AB materials. These results are expected to guide rational designs of Au@AB hybrid materials for different uses.

Light-driven dynamic Archimedes spirals and periodic oscillatory patterns of topological solitons in anisotropic soft matter.

Oscillatory and excitable systems commonly exhibit formation of dynamic non-equilibrium patterns. For example, rotating spiral patterns are observed in biological, chemical, and physical systems ranging from organization of slime mold cells to Belousov-Zhabotinsky reactions, and to crystal growth from nuclei with screw dislocations. Here we describe spontaneous formation of spiral waves and a large variety of other dynamic patterns in anisotropic soft matter driven by low-intensity light. The unstructured ambient or microscope light illumination of thin liquid crystal films in contact with a self-assembled azobenzene monolayer causes spontaneous formation, rich spatial organization, and dynamics of twisted domains and topological solitons accompanied by the dynamic patterning of azobenzene group orientations within the monolayer. Linearly polarized incident light interacts with the twisted liquid crystalline domains, mimicking their dynamics and yielding patterns in the polarization state of transmitted light, which can be transformed to similar dynamic patterns in its intensity and interference color. This shows that the delicate light-soft-matter interaction can yield complex self-patterning of both. We uncover underpinning physical mechanisms and discuss potential uses.

Billion-Fold Increase in Tip-Enhanced Raman Signal

A billion-fold increase in the Raman signal over conventional tip-enhanced Raman spectroscopy/microscopy (TERS) is reported. It is achieved by introducing a stimulating beam confocal with the pump beam into a conventional TERS setup. A stimulated TERS spectrum, closely corresponding to its spontaneous TERS counterpart, is obtained by plotting the signal intensity of the strongest Raman peak of an azobenzene thiol self-assembled monolayer versus the stimulating laser frequency. The stimulated TERS image of azobenzene thiol molecules grafted onto Au ⟨111⟩ clearly shows the surface distribution of the molecules, whereas, when compared to the simultaneously recorded surface topography, it presents an image contrast of different nature. The experimentally obtained stimulated gain is estimated at 1.0 × 10(9), which is in reasonable agreement with the theoretically predicted value. In addition to the signal increase, the signal-to-noise ratio was 3 orders of magnitude higher than in conventional spontaneous TERS. The proposed stimulated TERS technique offers the possibility for a substantially faster imaging of the surface with respect to normal TERS.

Voltage‐Controlled Nonvolatile Molecular Memory of an Azobenzene Monolayer through Solution‐Processed Reduced Graphene Oxide Contacts

The solution-processed fabrication of an azobenzene (ABC10) monolayer-based nonvolatile memory device on a reduced graphene oxide (rGO) electrode is successfully accomplished. Trans--cis isomerizations of ABC10 between two rGO electrodes in a crossbar device are controlled by applied voltage. An rGO soft-contact top electrode plays an important role in the conformational-change-dependent conductance switching process of an ABC10 monolayer.

Steric Hindrance of Photoswitching in Self-Assembled Monolayers of Azobenzene and Alkane Thiols

Surface-bound azobenzenes exhibit reversible photoswitching via trans-cis photoisomerization and have been proposed for a variety of applications such as photowritable optical media, liquid crystal displays, molecular electronics, and smart wetting surfaces. We report a novel synthetic route using simple protection chemistry to form azobenzene-functionalized SAMs on gold and present a mechanistic study of the molecular order, orientation, and conformation in these self-assembled monolayers (SAMs). We use vibrational sum-frequency generation (VSFG) to characterize their vibrational modes, molecular orientation, and photoisomerization kinetics. Trans-cis conformational change of azobenzene leads to the change in the orientation of the nitrile marker group detected by VSFG. Mixed SAMs of azobenzene and alkane thiols are used to investigate the steric hindrance effects. While 100% azobenzene SAMs do not exhibit photoisomerization due to tight packing, we observe reversible switching (>10 cycles) in mixed SAMs with only 34% and 50% of alkane thiol spacers.

Quantification of finite-temperature effects on adsorption geometries ofπ-conjugated molecules: Azobenzene/Ag(111)

The adsorption structure of the molecular switch azobenzene on Ag(111) is investigated by a combination of normal incidence x-ray standing waves and dispersion-corrected density functional theory. The inclusion of non-local collective substrate response (screening) in the dispersion correction improves the description of dense monolayers of azobenzene, which exhibit a substantial torsion of the molecule. Nevertheless, for a quantitative agreement with experiment explicit consideration of the effect of vibrational mode anharmonicity on the adsorption geometry is crucial.

Increased efficiency of light-emitting diodes incorporating anodes functionalized with fluorinated azobenzene monolayers and a green-emitting polyfluorene derivative

We investigate the functionalization of gold anodes with azobenzene-based self-assembled monolayers (AZO-SAM) and the influence of such functionalization on the external quantum efficiency (EQE) of polyfluorene-based light-emitting diodes (LEDs). Photoluminescence and electroluminescence measurements show that the AZO-SAMs do not modify the shape of the emission spectrum of the active layer. Instead, AZO-SAMs enhance the EQE of LEDs by an order of magnitude (from 0.018% to 0.18%) and decrease the turn-on voltage from 7.9 V to 6.2 V by reducing the injection barrier at the anode, thus promoting a better balance between hole and electron populations in the active layer.

First-principle-based MD description of azobenzene molecular rods

Extensive density functional theory (DFT) calculations have been performed to develop a force field for the classical molecular dynamics (MD) simulations of various azobenzene derivatives. Besides azobenzene, we focused on a thiolated azobenzene’s molecular rod (4′-{[(1,1′-biphenyl)-4-yl]diazenyl}-(1,1′-biphenyl)-4-thiol) that has been previously demonstrated to photoisomerize from trans to cis with high yields on surfaces. The developed force field is an extension of OPLS All Atoms, and key bonding parameters are parameterized to reproduce the potential energy profiles calculated by DFT. For each of the parameterized molecule, we propose three sets of parameters: one best suited for the trans configuration, one for the cis configuration, and finally, a set able to describe both at a satisfactory degree. The quality of the derived parameters is evaluated by comparing with structural and vibrational experimental data. The developed force field opens the way to the classical MD simulations of self-assembled monolayers (SAMs) of azobenzene’s molecular rods, as well as to the quantum mechanics/molecular mechanics study of photoisomerization in SAMs.

Raman spectroscopy and polarization: Selected case studies

We show, through several selected case studies, the potential benefits that can be obtained by controlling the polarization states of the exciting and scattered radiations in a Raman scattering experiment. When coupled with polarization control, Raman spectroscopy is thus capable of providing extra information on the structural properties of the materials under investigation. The experimental examples presented in this work are taken from the area of both conventional, i.e., far-field, as well as from near-field Raman spectroscopy. They cover topics such as the stress tensor measurement in strained semiconductor structures, the vibration mode assignment in pentacene thin films and the Raman scattering tensor determination from near-field measurements on azobenzene monolayers. The basic theory necessary for modelling the far- and near-field polarized Raman responses is also given and the model efficiency is illustrated on the experimental data.

Electronic and transport properties of azobenzene monolayer junctions as molecular switches

We investigate from first principles the change in transport properties of a two-dimensional azobenzene monolayer sandwiched between two Au electrodes that undergoes molecular switching. We focus on transport differences between a chemisorbed and physisorbed top monolayer-electrode contact. The conductance of the monolayer junction with a chemisorbed top contact is higher in the trans configuration, in agreement with the previous theoretical predictions of one-dimensional single-molecule junctions. However, with a physisorbed top contact, the ON state, with larger conductance, is associated with the cis configuration due to a reduced effective tunneling pathway, which successfully explains recent experimental measurements on azobenzene monolayer junctions. A simple model is developed to explain electron transmission across subsystems in the molecular junction. We also discuss the effects of monolayer packing density, molecule tilt angle, and contact geometry on the calculated transmission functions. In particular, we find that a tip-like contact with chemisorption significantly affects the electric current through the cis monolayer, leading to highly asymmetric current-voltage characteristics as well as large negative differential resistance behavior.