Samuel Graham

A Universal Method to Produce Low–Work Function Electrodes for Organic Electronics

A Sturdy Electrode Coating To operate efficiently, organic devices—such as light-emitting diodes—require electrodes that emit or take up electrons at low applied voltages (that is, have low work functions). Often these electrodes are metals, such as calcium, that are not stable in air or water vapor and have to be protected from environmental damage. Zhou et al. (p. 327; see the Perspective by Helander) report that a coating polymer containing aliphatic amine groups can lower the work functions of various types of electrodes by up to 1.7 electron volts and can be used in a variety of devices. Air-stable, physisorbed polymers containing aliphatic amine groups can improve the efficiency of organic electronic devices. Organic and printed electronics technologies require conductors with a work function that is sufficiently low to facilitate the transport of electrons in and out of various optoelectronic devices. We show that surface modifiers based on polymers containing simple aliphatic amine groups substantially reduce the work function of conductors including metals, transparent conductive metal oxides, conducting polymers, and graphene. The reduction arises from physisorption of the neutral polymer, which turns the modified conductors into efficient electron-selective electrodes in organic optoelectronic devices. These polymer surface modifiers are processed in air from solution, providing an appealing alternative to chemically reactive low–work function metals. Their use can pave the way to simplified manufacturing of low-cost and large-area organic electronic technologies.

Electrical, Thermal, and Mechanical Characterization of Silicon Microcantilever Heaters

Silicon atomic force microscope (AFM) cantilevers having integrated solid-state heaters were originally developed for application to data storage, but have since been applied to metrology, thermophysical property measurements, and nanoscale manufacturing. These applications beyond data storage have strict requirements for mechanical characterization and precise temperature calibration of the cantilever. This paper describes detailed mechanical, electrical, and thermal characterization and calibration of AFM cantilevers having integrated solid-state heaters. Analysis of the cantilever response to electrical excitation in both time and frequency domains aids in resolving heat transfer mechanisms in the cantilever. Raman spectroscopy provides local temperature measurement along the cantilever with resolution near 1 mum and 5degC and also provides local surface stress measurements. Observation of the cantilever mechanical thermal noise spectrum at room temperature and while heated provides insight into cantilever mechanical behavior and compares well with finite-element analysis. The characterization and calibration methodology reported here expands the use of heated AFM cantilevers, particularly the uses for nanomanufacturing and sensing

Pharmacological Characterization of MK-0974 [N-[(3R,6S)-6-(2,3-Difluorophenyl)-2-oxo-1-(2,2,2-trifluoroethyl)azepan-3-yl]-4-(2-oxo-2,3-dihydro-1H-imidazo[4,5-b]pyridin-1-yl)piperidine-1-carboxamide], a Potent and Orally Active Calcitonin Gene-Related Peptide Receptor Antagonist for the Treatment of Migraine

Calcitonin gene-related peptide (CGRP) is a potent neuropeptide that plays a key role in the pathophysiology of migraine headache. CGRP levels in the cranial circulation are increased during a migraine attack, and CGRP itself has been shown to trigger migraine-like headache. The correlation between CGRP release and migraine headache points to the potential utility of CGRP receptor antagonists as novel therapeutics in the treatment of migraine. Indeed, clinical proof-of-concept in the acute treatment of migraine was demonstrated with an intravenous formulation of the CGRP receptor antagonist BIBN4096BS (olcegepant). Here we report on the pharmacological characterization of the first orally bioavailable CGRP receptor antagonist in clinical development, MK-0974 [N-[(3R,6S)-6-(2,3-difluorophenyl)-2-oxo-1-(2,2,2-trifluoroethyl)azepan-3-yl]-4-(2-oxo-2,3-dihydro-1H-imidazo[4,5-b]pyridin-1-yl)piperidine-1-carboxamide]. In vitro, MK-0974 is a potent antagonist of the human (Ki = 0.77 nM) and rhesus (Ki = 1.2 nM) CGRP receptors but displays >1500-fold lower affinity for the canine and rat receptors as determined via 125I-human CGRP competition binding assays. A rhesus pharmacodynamic assay measuring capsaicin-induced changes in forearm dermal blood flow via laser Doppler imaging was utilized to determine the in vivo activity of CGRP receptor antagonism. MK-0974 produced a concentration-dependent inhibition of dermal vasodilation, generated by capsaicin-induced release of endogenous CGRP, with plasma concentrations of 127 and 994 nM required to block 50 and 90% of the blood flow increase, respectively. In conclusion, MK-0974 is a highly potent, selective, and orally bioavailable CGRP receptor antagonist, which may be valuable in the acute treatment of migraine.

First Demonstration of Cerebrospinal Fluid and Plasma Aβ Lowering with Oral Administration of a β-Site Amyloid Precursor Protein-Cleaving Enzyme 1 Inhibitor in Nonhuman Primates

β-Site amyloid precursor protein (APP)-cleaving enzyme (BACE) 1 cleavage of amyloid precursor protein is an essential step in the generation of the potentially neurotoxic and amyloidogenic Aβ42 peptides in Alzheimers disease. Although previous mouse studies have shown brain Aβ lowering after BACE1 inhibition, extension of such studies to nonhuman primates or man was precluded by poor potency, brain penetration, and pharmacokinetics of available inhibitors. In this study, a novel tertiary carbinamine BACE1 inhibitor, tertiary carbinamine (TC)-1, was assessed in a unique cisterna magna ported rhesus monkey model, where the temporal dynamics of Aβ in cerebrospinal fluid (CSF) and plasma could be evaluated. TC-1, a potent inhibitor (IC50 ∼ 0.4 nM), has excellent passive membrane permeability, low susceptibility to P-glycoprotein transport, and lowered brain Aβ levels in a mouse model. Intravenous infusion of TC-1 led to a significant but transient lowering of CSF and plasma Aβ levels in conscious rhesus monkeys because it underwent CYP3A4-mediated metabolism. Oral codosing of TC-1 with ritonavir, a potent CYP3A4 inhibitor, twice daily over 3.5 days in rhesus monkeys led to sustained plasma TC-1 exposure and a significant and sustained reduction in CSF sAPPβ, Aβ40, Aβ42, and plasma Aβ40 levels. CSF Aβ42 lowering showed an EC50 of ∼20 nM with respect to the CSF [TC-1] levels, demonstrating excellent concordance with its potency in a cell-based assay. These results demonstrate the first in vivo proof of concept of CSF Aβ lowering after oral administration of a BACE1 inhibitor in a nonhuman primate.

Invited Article: Simultaneous mapping of temperature and stress in microdevices using micro-Raman spectroscopy.

Analysis of the Raman Stokes peak position and its shift has been frequently used to estimate either temperature or stress in microelectronics and microelectromechanical system devices. However, if both fields are evolving simultaneously, the Stokes shift represents a convolution of these effects, making it difficult to measure either quantity accurately. By using the relative independence of the Stokes linewidth to applied stress, it is possible to deconvolve the signal into an estimation of both temperature and stress. Using this property, a method is presented whereby the temperature and stress were simultaneously measured in doped polysilicon microheaters. A data collection and analysis method was developed to reduce the uncertainty in the measured stresses resulting in an accuracy of +/-40 MPa for an average applied stress of -325 MPa and temperature of 520 degrees C. Measurement results were compared to three-dimensional finite-element analysis of the microheaters and were shown to be in excellent agreement. This analysis shows that Raman spectroscopy has the potential to measure both evolving temperature and stress fields in devices using a single optical measurement.

Development of a thermal resistance model for chip-on-board packaging of high power LED arrays

Abstract The performance of high power LEDs strongly depends on the junction temperature. Operating at high junction temperature causes degradation of light intensity and lifetime. Therefore, proper thermal management is critical for LED packaging. While the design of the heat sink is a major contributor to lowering the overall thermal resistance of the packaged luminaire, another area of concern arises from the need to address the large heat fluxes that exist beneath the die. In this study we conduct a thermal analysis of high power LED packages implementing chip-on-board (COB) architecture combined with power electronic substrate focusing on heat spreading effect. An analytical thermal resistance model is presented for the LED array and validated by comparing it with finite element analysis (FEA) results. By using the analytical expression of thermal resistance, it is possible to understand the impact of design parameters (e.g., material properties, LED spacing, substrate thickness, etc.) on the package thermal resistance, bypassing the need for detailed computational simulations using FEA.

A hybrid encapsulation method for organic electronics

We report a thin-film encapsulation method for organic electronics that combines the deposition of a layer of SiOx or SiNx (100 nm) by plasma enhanced chemical vapor deposition followed by a layer of Al2O3 (10–50 nm) by atomic layer deposition and a 1-μm-thick layer of parylene by chemical vapor deposition. The effective water vapor transmission rates of the encapsulation was (2±1)×10−5 g/m2 day at 20 °C and 50% relative humidity (RH). The encapsulation was integrated with pentacene/C60 solar cells, which showed no decrease in conversion efficiency after 5800 h of exposure to air demonstrating the effectiveness of the encapsulation methodology.

Controlled Doping of Large‐Area Trilayer MoS2 with Molecular Reductants and Oxidants

Highly uniform large-area MoS2 is chemically doped using molecular reductants and oxidants. Electrical measurements, photoemission, and Raman spectroscopy are used to study the doping effect and to understand the underlying mechanism. Strong work-function changes of up to ±1 eV can be achieved, with contributions from state filling and surface dipoles. This results in high doping densities of up to ca. 8 × 10(12) cm(-2) .

Influence of Interfacial Mixing on Thermal Boundary Conductance Across a Chromium/Silicon Interface

The thermal conductance at solid-solid interfaces is becoming increasingly important in thermal considerations dealing with devices on nanometer length scales. Specifically, interdiffusion or mixing around the interface, which is generally ignored, must be taken into account when the characteristic lengths of the devices are on the order of the thickness of this mixing region. To study the effect of this interfacial mixing on thermal conductance, a series of Cr films is grown on Si substrates subject to various deposition conditions to control the growth around the Cr/Si boundary. The Cr/Si interfaces are characterized with Auger electron spectroscopy. The thermal boundary conductance (h BD ) is measured with the transient thermoreflectance technique. Values of h BD are found to vary with both the thickness of the mixing region and the rate of compositional change in the mixing region. The effects of the varying mixing regions in each sample on h BD are discussed, and the results are compared to the diffuse mismatch model (DMM) and the virtual crystal DMM (VCDMM), which takes into account the effects of a two-phase region of finite thickness around the interface on h BD . An excellent agreement is shown between the measured h BD and that predicted by the VCDMM for a change in thickness of the two-phase region around the interface.

A practical extension of the 3ω method to multilayer structures

An improved data reduction method is used to extend the popular 3ω method to general layered geometries. This approach utilizes an unapproximated analytical solution, cast in terms of thermal impedance, to simultaneously measure thermal conductivity, thermal capacity, conductivity anisotropy, and interlayer contact resistance in multilayer planar structures. This method places no restrictions on the number or thickness of individual material layers, and it allows experimental measurements to be taken over a much wider range of frequencies than was previously possible. The search algorithm associated with the model is straightforward, robust, and requires no specialized software to compose. Experimental results are presented for a two-layer borosilicate glass/zeolite structure as well as a Si∕SiO2 structure. In both instances, the algorithm was able to simultaneously extract thermal conductivity and thermal diffusivity values using a single series of 3ω measurements.