We demonstrate tuning of Schottky energy barriers in organic electronic devices by utilizing chemically tailored electrodes. The Schottky energy barrier of Ag on poly͓2-methoxy, 5-͑2Ј-ethyl-hexyloxy͒-1,4-phenylene was tuned over a range of more than 1 eV by using self-assembled monolayers ͑SAM's͒ to attach oriented dipole layers to the Ag prior to device fabrication. Kelvin probe measurements were used to determine the effect of the SAM's on the Ag surface potential. Ab initio Hartree-Fock calculations of the molecular dipole moments successfully describe the surface potential changes. The chemically tailored electrodes were then incorporated in organic diode structures and changes in the metal/organic Schottky energy barriers were measured using an electroabsorption technique. These results demonstrate the use of selfassembled monolayers to control metal/organic interfacial electronic properties. They establish a physical principle for manipulating the relative energy levels between two materials and demonstrate an approach to improve metal/organic contacts in organic electronic devices.
Vision and other light‐triggered biochemical transformations in plants and living organisms represent a sophisticated biological processes in which optical signals are recorded and transduced as (physico)chemical events. Photoswitchable biomaterials are a new class of substances in which optical signals generate discrete “On” and “Off” states of biological functions, resembling logic gates that flip between 0 and 1 states in response to the changes in electric currents in computers. The (photo)chemistry of photochromic materials has been extensively developed in the past four decades. These materials isomerize reversibly upon light absorption, and the discrete photoisomeric states exhibit distinct spectral and chemical features. Integration of photoisomerizable (or photochromic) units into biomaterials allow their secondary functions such as biocatalysis, binding, and electron transfer to be tailored so that they can be switched on or off. This can be accomplished by chemical modification of the biomaterial by photoisomerizable units and by integration of biomaterials in photoisomerizable microenvironments such as monolayers or polymers. The photoswitchable properties of chemically modified biomaterials originate from the light‐induced generation or perturbation of the biologically active site, whereas in photoisomerizable matrices they depend upon the regulation of the physical or chemical features of the photoisomerizable assemblies of polymers, monolayers, or membranes. Light‐triggered activation of catalytic biomaterials provides a means of amplifying the recorded optical signal by biochemical transformations, and photostimulated biochemical redox switches allow its electrochemical transduction and amplification. The field of photoswitches based on biomaterials has developed extensively in the past few years within the general context of molecular switching devices and micromachinery. The extensive knowledge on the manipulation of biomaterials through genetic engineering and the fabrication of surfaces modified by biologically active materials enables us to prepare biomaterials with improved optical‐switching features. Their application in optoelectronic or bioelectronic devices has been transformed from fantasy to reality. The use of photoswitchable biomaterials in information storage and processing devices (biocomputers), sensors, reversible immunosensors, and biological amplifiers of optical signals has already been demonstrated, but still leaves important future challenges.
We describe a novel approach for Au and Ag colloid monolayer formation on different silicon oxide surfaces such as glass, silicon, and ITO. The preparation method is simple and yields monolayers with easily controlled spacing within the monolayer without aggregation of metal particles. The colloid monolayers are prepared in two steps: (1) modification of the substrates with starburst dendrimers and (2) noble metal colloid deposition onto the dendrimer layer. Different Au and Ag colloids, ranging from 15 to 80 nm in particle diameter, have been deposited onto the dendrimer-modified surfaces. The structure and properties of the resulting particle arrays have been studied by atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), UV-vis spectroscopy, and surface-enhanced Raman scattering (SERS). XPS data show that the dendrimers spontaneously adsorb to various silicon oxide surfaces. The thickness of the dendrimer overlayer has been calculated and lies in the d ) 14-25 Å range. SEM and AFM data show that the colloids spontaneously form continuous films on the dendrimermodified surfaces. The noble metal particles are well isolated and confined to a single layer, and aggregation does not occur on the surface. The interparticle spacing (74-829 nm) and surface coverage can be controlled over a wide range by colloid size, colloid concentration, and immersion time. UV-vis spectroscopic data show that the microstructure directly controls the optical properties of the layer. Finally, we demonstrate that the prepared substrates provide a useful platform for SERS studies of materials adsorbed on the metal particles.
Several patterned monolayers of alkanethiols
CH3(CH2)
n
-
1SH
on a polycrystalline Au substrate were
prepared by using microcontact printing and solution deposition
methods, and their surfaces were examined
by IR spectroscopy, scanning force microscopy, lateral force microscopy
(LFM), and force modulation
microscopy (FMM). Our work shows that LFM and FMM can detect
differences in packing density of
chemically identical molecules which are too small to be detected by
IR, ellipsometry, and wetting
measurements and suggests that the tip−sample contact area is an
important parameter governing the
contrasts of LFM and FMM images. Stiffness images obtained with
FMM depend on changes in the
Young's modulus of a sample surface as well as in the tip−sample
contact area. As a result, a surface
region of small modulus can have a large stiffness due to its large
contact area.
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