Matthew Farrow

University of Colorado at Boulder , Boulder, CO, United States

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Publications (6)13.3 Total impact

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    ABSTRACT: The initial development of photoinduced anisotropy in highly photosensitive monolayers of an aminoazobenzene molecule (dMR, a derivative of o-methyl red) that are initially randomized using circularly polarized light is found to be significantly slower than in monolayers randomized by thermal relaxation. We propose that this is a direct consequence of the slow thermal relaxation of isomers from the cis to the trans state and suggest that such considerations are important in designing even more sensitive photoactive monolayers and in understanding their photodynamics.
    Journal of Applied Physics 05/2011; 109(10):103521-103521-5. · 2.19 Impact Factor
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    ABSTRACT: Photosensitive surfaces treated to have in-plane structural anisotropy by illumination with polarized light can be used to orient liquid crystals (LCs). Here we report a detailed study of the dynamic behavior of this process at both short and long times, comparing the ordering induced in the bare active surface with that of the LC in contact with the surface using a high-sensitivity polarimeter that enables detailed characterization of the anisotropy of the active surface. The experiments were carried out using self-assembled monolayers (SAMs) made from dimethylaminoazobenzene covalently bonded to a glass surface through a triethoxysilane terminus. This surface gives planar alignment of the liquid crystal director with an azimuthal orientation that can be controlled by the polarization of actinic light. We find a remarkable long-term collective interaction between the orientationally ordered SAM and the director field of the LC: while an azobenzene based SAM in contact with an isotropic gas or liquid relaxes to an azimuthally isotropic state in the absence of light due to thermal fluctuations, an orientationally written SAM in contact with LC in the absence of light can maintain the LC director twist permanently, that is, the SAM is capable of providing azimuthal anchoring to the LC even in the presence of a torque about the surface normal. We find that the short-time, transient LC reorientation is limited by the weak azimuthal anchoring strength of the SAM and by the LC viscosity.
    Langmuir 10/2010; 26(22):17482-8. · 4.38 Impact Factor
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    ABSTRACT: We describe a new type of optically controlled liquid crystal alignment layer that demonstrates unprecedented performance. It consists of an aminoazobenzene-type material with a very simple molecular structure, which is derived from methyl red by a one-step synthesis. We have devised a method of forming covalently attached monolayers of this material on glass by an amine-assisted condensation reaction involving the triethoxysilane end of the molecule. A nematic liquid crystal (LC) cell made with the monolayer and a rubbed polymer layer was switched from a uniform state to a twisted state with a polarized 450 nm control beam having a dose of 5.5 mJ/cm(2). This is equivalent to an average of only one absorbed photon per azobenzene group. Through atomic force microscopy, absorption spectroscopy, spectroscopic ellipsometry, and second harmonic generation experiments, we have confirmed that layers of this type are smooth and uniform with a surface coverage consistent with a monolayer and that the azobenzene groups are tilted, on average, 55 degrees with respect to the surface normal. These characteristics lead to a large interaction energy density between the layer and LC. The monolayer's rapid response in developing anisotropy in this property can be attributed to a large absorption cross section, as well as the favorable tilt angle, which allows for sufficient photoisomerization free volume in a dense layer.
    Langmuir 01/2009; 25(2):997-1003. · 4.38 Impact Factor
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    ABSTRACT: Self-Assembled Monolayers (SAMs) synthesized on glass and incorporating azobenzene are illuminated with green light at normal incidence to study the effect of photo-isomerization on in-plane molecular orientation. Measurements of the monolayer birefringence at the glass-gas interface show that the SAM orientation and order parameter dynamics are subdiffusive, characterized by a stretched exponential relaxation with a distribution of relaxation times. Order decays with power law relaxation and exponents that decrease with increased initial writing intensity and/or duration, indicative of orientational trapping wells with a distribution of depths. Results on dynamics of reorientation at the glass-solvent interface will also be presented. This work was supported by NSF Grant CHE-0079122 and NSF MRSEC Grant DMR-0213918.
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    ABSTRACT: A simple procedure for the preparation of octadecylsiloxane self-assembled monolayers (SAMs) on float glass substrates is described. The method utilizes commercial octadecyltriethoxysilane, OTE: n-C18H37Si(OCH2CH3)3, as the SAM precursor, with deposition accomplished in toluene solution using n-butylamine as catalyst. This synthetic approach obviates the use of the problematic trichlorosilanes typically required for the preparation of high quality SAMs, and is characterized by a wide ‘process window,’ utilizing off-the-shelf reagents without special handling.
    Liquid Crystals 04/2004; 31(4):481-489. · 2.35 Impact Factor
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    ABSTRACT: We have prepared Self-Assembled Monolayers (SAMs) incorporating azobenzenes to study the effect of photo-isomerization on liquid crystal (LC) alignment. To study the effects of irradiating the SAMs with polarized UV light, we have carried out in situ tests of transmission of nematic cells which consist of one fresh azo-SAM substrate and one rubbed nylon substrate. Cells are initially uniform, with the director aligning parallel to the nylon rubbing direction. When illuminated with UV light polarized parallel to the rubbing direction, the LC orientation at the SAM surface rotates gradually through 90^circ and the cell becomes twisted. The reorientation occurs more quickly when the UV intensity is increased. The long axes of the LC molecules at the SAM boundary thus align perpendicular to the polarization direction of the UV light. The alignment effect disappears gradually when the sample is heated to approximately 120^circC. This work is supported by NSF grant CHE-0079122 and NSF MRSEC grant DMR-9809555.