The electrical behavior of nitro oligo(phenylene ethynylene)’s in pure and mixed monolayers

In order to realize molecular electronic devices, molecules with electrically interesting behavior must be identified. One molecule that has potential for use in devices is an oligo(phenylene ethynylene) (OPE) molecule with nitro sidegroup(s). These “nitro” molecules have been reported to show electrical switching with memory behavior, as well as negative differential resistance (NDR). However, different research groups testing the nitro molecules in different test beds have observed different electrical behaviors. In this work, we assembled two different nitro monolayers: one completely composed of nitro molecules and the second a mixed matrix where nitro molecules were separated by dodecanethiol molecules. We used scanning tunneling microscopy to image each of the monolayers and observed that the nitro molecules were effectively inserted into the ordered dodecanethiol monolayer. We tested the electrical behavior of the pure monolayer, as well as the mixed monolayer, in our nanowell test device. The nanowell devices were fabricated on micron-size gold lines patterned on oxide-coated silicon wafers. The gold lines were covered with a silicon dioxide layer, through which a nanometer size well was milled. This nanowell device was filled with a self-assembling monolayer of organic molecules, and capped with titanium and gold. The nanowell electrical results showed switching with memory for the pure nitro monolayer, but not for the mixed monolayer. This switching behavior consisted of a molecule starting in a high conductivity state and switching to a low conductivity state upon application of a threshold voltage. The high conductivity state could only be returned by application of an opposite threshold voltage.     

Polymeric Light‐Emitting Diodes Based on Poly(p‐phenylene ethynylene), Poly(triphenyldiamine), and Spiroquinoxaline

In this paper polymeric light‐emitting diodes (LEDs) based on alkoxy‐substituted poly(p‐phenylene ethynylene) EHO‐OPPE as emitter material in combination with poly(triphenyldiamine) as hole transport material are demonstrated. Different device configurations such as single‐layer devices, two‐layer devices, and blend devices were investigated. Device improvement and optimization were obtained through careful design of the device structure and composition. Furthermore, the influence of an additional electron transporting and hole blocking layer (ETHBL), spiroquinoxaline (spiro‐qux), on top of the optimized blend device was investigated using a combinatorial method, which allows the preparation of a number of devices characterized by different layer thicknesses in one deposition step. The maximum brightness of the investigated devices increased from 4 cd/m² for a device of pure EHO‐OPPE to 260 cd/m² in a device with 25 % EHO‐OPPE + 75 % poly(N,N′‐diphenylbenzidine diphenylether) (poly‐TPD) as the emitting/hole‐transporting layer and an additional electron‐transport/hole‐blocking spiro‐qux layer of 48 nm thickness.     

α,ω‐Dithiol Oligo(phenylene vinylene)s for the Preparation of High‐Quality π‐Conjugated Self‐Assembled Monolayers and Nanoparticle‐ Functionalized Electrodes

While thioacetate‐terminated oligo(phenylene vinylene)s (OPVs) have been synthesized and employed in applications involving the formation of metal–molecule–metal junctions, the synthesis and application of potentially more versatile α,ω‐dithiol OPVs have not previously been described. Here, a thiomethyl‐precursor route to the synthesis of α,ω‐dithiol OPVs is reported and their ability to form well‐ordered self‐assembled monolayers (SAMs) without the addition of exogenous deprotection reagents is described. α,ω‐Dithiol OPV monolayers exhibit thicknesses consistent with molecular length and are nearly defect‐free, as assayed by electrochemical measurements. To demonstrate the ease with which SAMs containing these bifunctional OPVs can, in contrast to thioacetate functionalized OPVs, be further functionalized with materials other than gold, we have modified them in a single step with a sub‐monolayer of cadmium selenide nanocrystals (NCs). Scanning electron microscopy (SEM) and atomic force microscopy (AFM) confirm that these NC‐modified films are both smooth and uniform over the largest areas investigated (> 10 μm²) and no evidence of NC aggregation is observed. To evaluate the electrochemical response of these metal–molecule–semiconductor assemblies we have fabricated NC‐modified OPV SAMs with ferrocene‐coated NCs. Variable‐frequency alternating current voltammetry indicates that electron transfer in these assemblies is much more rapid than in analogous structures formed using simple alkane dithiols. It thus appears that α,ω‐dithiol OPVs are well suited for the formation of high‐quality conducting SAMs for the functionalization of gold and other surfaces.     

Branched Oligo(ether) Side Chains: A Path to Enhanced Processability and Elevated Conductivity for Polymeric Semiconductors

Commercialization of stable conjugated polymers (CPs) with tunable electronic properties will remain a challenge without adequate solution processability due to the importance of techniques such as roll-to-roll manufacturing. Consequently, modifying CP backbones with polar side chains has recently resurged as an attractive structural design approach to improve polymer solubility and to provide CPs with the capability of transporting both electrons and ions, which is crucial for applications such as organic electrochemical transistors (OECTs). Here, a new dioxythiophene copolymer comprised of 2,​2'-bis-(3,4-ethylenedioxy)thiophene (biEDOT) and 3,4-propylenedioxythiophene (ProDOT) substituted with branched oligo(ether) side chains (PE₂-biOE2OE3) is synthesized using two direct hereto(arylation) polymerization (DHAP) techniques. The typical DHAP technique results in a lower molecular weight polymer (PE₂-biOE2OE3(L)), which is soluble in acetone and demonstrated a solid-state conductivity after oxidative doping of 55 ± 3 S cm⁻¹. Alternatively, a unique temperature ramp DHAP methodology results in a higher molecular weight polymer (PE₂-biOE2OE3(H)) with an especially high solid-state conductivity of 430 ± 60 S cm⁻¹. Notably, the first OECT fabricated from an acetone-processed polymer is reported, which is stable up to 500 cycles and can provide a pathway for future material design aimed at eliminating the use of toxic chlorinated solvents in OECT active layer processing.     

Surface Structure of Molecular Beam Epitaxy (211)B HgCdTe

The as-grown molecular beam epitaxy (MBE) (211)B HgCdTe surface has variable surface topography, which is primarily dependent on substrate temperature and substrate/epilayer mismatch. Nano-ripple formation and cross-hatch patterning are the predominant structural features observed. Nano-ripples preferentially form parallel to the \( [\bar {1}11] \) and are from 0 Å to 100 Å in height with a wavelength between 0.1 μm and 0.8 μm. Cross-hatch patterns result from slip dislocations in the three {111} planes intersecting the (211) growth surface. The cross-hatch step height is 4 ± 1 Å (limited data set). This indicates that only a bi-layer slip (Hg/Cd + Te) in the {111} slip plane occurs. For the deposition of MBE (211)B HgCdTe/CdTe/Si, the reorientation of multiple nano-ripples coalesced into “packets” forms cross-hatch patterns. The as-grown MBE (211)B CdTe/Si surface is highly variable but displays nano-ripples and no cross-hatch pattern. Three types of defects were observed by atomic force microscopy (AFM): needle, void/hillock, and voids.     

Structural properties of GaAsN grown on (001) GaAs by metalorganic molecular beam epitaxy

Detailed transmission electron microscopy (TEM) and transmission electron diffraction (TED) examination has been made of metalorganic molecular beam epitaxial GaAsN layers grown on (001) GaAs substrates. TEM results show that lateral composition modulation occurs in the GaAs_1−xN_x layer (x 6.75%). It is shown that increasing N composition and Se (dopant) concentration leads to poor crystallinity. It is also shown that the addition of Se increases N composition. Atomic force microscopy (AFM) results show that the surfaces of the samples experience a morphological change from faceting to islanding, as the N composition and Se concentration increase. Based on the TEM and AFM results, a simple model is given to explain the formation of the lateral composition modulation.     

Compositional Dependence of the Performance of Poly(p‐phenylene vinylene):Methanofullerene Bulk‐Heterojunction Solar Cells

The dependence of the performance of OC₁C₁₀‐PPV:PCBM (poly(2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐p‐phenylene vinylene):methanofullerene [6,6]‐phenyl C₆₁‐butyric acid methyl ester)‐based bulk heterojunction solar cells on their composition has been investigated. With regard to charge transport, we demonstrate that the electron mobility gradually increases on increasing the PCBM weight ratio, up to 80 wt.‐%, and subsequently saturates to its bulk value. Surprisingly, the hole mobility in the PPV phase shows an identical behavior and saturates beyond 67 wt.‐% PCBM, a value which is more than two orders of magnitude higher than that of the pure polymer. The experimental electron and hole mobilities were used to study the photocurrent generation of OC₁C₁₀‐PPV:PCBM bulk‐heterojunction (BHJ) solar cells. From numerical calculations, it is shown that for PCBM concentrations exceeding 80 wt.‐% reduced light absorption is responsible for the loss of device performance. From 80 to 67 wt.‐%, the decrease in power conversion efficiency is mainly due to a decreased separation efficiency of bound electron–hole (e–h) pairs. Below 67 wt.‐%, the performance loss is governed by a combination of a reduced generation rate of e–h pairs and a strong decrease in hole transport.     

热转化温度对PPV的光致发光光谱的影响

在１４０－２７０℃下热转化２ｈ得到了ＰＰＶ，发现随着热转化温度的升高，ＰＰＶ的紫外－可见吸收峰和光致发光峰发生红移，发光强度降低，发光峰数由２个变为１个。通过与ＰＰＶ齐聚物的光致发光光谱的对比，提出５００－５３０ｎｍ峰是短共轭链段的一个光致发光峰的假设。     

Study of interaction between silicon surfaces in dilute ammonia peroxide mixtures (APM) and their components using atomic force microscope (AFM)

Force measurements have been conducted between H-terminated Si surface and Si tip in DI-water, NH₄OH:H₂O (1:100), H₂O₂:H₂O (1:100) and NH₄OH:H₂O₂:H₂O (1:1:100-1:1:500) solutions as a function of immersion time using atomic force microscopy (AFM). The approach force curve results show attractive forces in DI-water, NH₄OH:H₂O (1:100) and H₂O₂:H₂O (1:100) solutions at separation distances of less than 10 nm for all immersion times (2, 10 and 60 min) investigated in this study. In the case of dilute ammonia-hydrogen peroxide mixtures, the interaction forces are purely repulsive within 2 min of immersion time.The adhesion forces have also been measured between the surface and the tip in DI-water, NH₄OH:H₂O (1:100) and H₂O₂:H₂O (1:100) solutions. The magnitude of the adhesion force is in the range of 0.8-10.5 nN in these solutions. In dilute APM solutions, no adhesion force is measured between the surface and the tip and repulsive forces dominated at all separation distances.     

Comparison of dimethyl sulfoxide treated highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) electrodes for use in indium tin oxide-free organic electronic photovoltaic devices

Indium tin oxide (ITO)-free organic photovoltaic (OPV) devices were fabricated using highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the transparent conductive electrode (TCE). The intrinsic conductivity of the PEDOT:PSS films was improved by two different dimethyl sulfoxide (DMSO) treatments – (i) DMSO was added directly to the PEDOT:PSS solution (PEDOT:PSS_ADD) and (ii) a pre-formed PEDOT:PSS film was immersed in DMSO (PEDOT:PSS_IMM). X-ray photoelectron spectroscopy (XPS) and conductive atomic force microscopy (CAFM) studies showed a large amount of PSS was removed from the PEDOT:PSS_IMM electrode surface. OPV devices based on a poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM) bulk hetrojunction showed that the PEDOT:PSS_IMM electrode out-performed the PEDOT:PSS_ADD electrode, primarily due to an increase in short circuit current density from 6.62 mA cm⁻² to 7.15 mA cm⁻². The results highlight the importance of optimising the treatment of PEDOT:PSS electrodes and demonstrate their potential as an alternative TCE for rapid processing and low-cost OPV and other organic electronic devices.     

Surface structure of (111)A HgCdTe

The surface of (111)A HgCdTe has been studied by reflection high-energy electron diffraction and atomic force microscopy (AFM). The as-grown liquid-phase epitaxy (LPE) surface has bilayer (3.7 ± 0.2 Å) step/terrace structures, macro-steps, and cross-hatch patterns. Macro-steps occur about the $[11\bar 2]$ and are from 10–40 Å in height. AFM and x-ray measurements indicate the as-grown epilayer is ≈0.2° off-cut (random polar angle) from the (111). 〈110〉 cross-hatch lines consistent with bilayer (step height=3.9 ± 0.2 Å) {111} slip dislocation are observed. The native oxide/carbon layer for the as-grown LPE (111)A HgCdTe is ≈8 Å. The experimental results suggest that the as-grown LPE surface approximates an equilibrium vicinal crystal structure. The 0.1% Br:ethylene glycol wet chemically etched surfaces retained the macro-step structure, but numerous small protrusions (10–100 Å height, ≈300 Å diameter) developed. The plasma-etched (111)A HgCdTe surface is crystalline, but exhibits surface disorder and is roughened.     

复合自组装分子膜的摩擦特性研究

本文采用自组装技术制备了三氯十八硅烷(octadecyltrichorosilane 0TS)／3-胺基丙基-三甲氧基硅炕(3-amino-propyltrimethoxysilane APTMS)和APTMS／OTS复合自组装分子膜,在原子力／摩擦力显微镜上对薄膜的摩擦特性进行了测试,并与0TS和APTMS自组装分子膜(self-assembledmonolayers SAMs)进行了对比。结果表明,OTS／APTMS复合自组装分子膜因既保持了一定的键合强度叉增加了自组装分子的流动性,使其摩擦力显著降低。复合自组装分子膜的摩擦力随着载荷和滑动速度的增大而增大,这与自组装分子的受力响应和弛豫特性相关。合理地设计自组装分子膜可有效地减小摩擦。     

Optical properties and structure of InAs quantum dots in near-infrared band

The InAs quantum dots (QDs) grown by molecular beam epitaxy (MBE) are studied as a function of growth temperature at a specific InAs coverage of 2.7 ML. The QDs density is significantly reduced from 8.0 × 1010 to 5.0 × 109 cm-2 as the growth temperature increases from 480℃ to 520℃, while the average QDs diameter and height becomes larger. The effects of the growth temperature on the evolution of bimodal QDs are investigated by combining atomic force microscopy (AFM) and photoluminescence (PL). Results show that the formation of the bimodal QDs depends on the growth temperature: at a growth temperature of 480℃,large QDs result from the small QDs coalition; at a growth temperature of 535℃, the indium desorption and InAs segregation result in the formation of small QDs.     

Topography and Dislocations in (112)B HgCdTe/CdTe/Si

Scanning electron microscopy (SEM), atomic force microscopy (AFM), and x-ray diffraction (XRD) measurements all indicate an approximate factor of ten increase in the Everson etch pit density (EPD) over standard Nomarski microscopy Everson EPD determination. A new (112)B CdTe/Si EPD etch has also been demonstrated which reduces the surface roughness of the etched epilayer and makes etch pit density determination less problematic.     

High‐Performance Solution‐Processable Poly(p‐phenylene vinylene)s for Air‐Stable Organic Field‐Effect Transistors

The influence of the substitution pattern (unsymmetrical or symmetrical), the nature of the side chain (linear or branched), and the processing of several solution processable alkoxy‐substituted poly(p‐phenylene vinylene)s (PPVs) on the charge‐carrier mobility in organic field‐effect transistors (OFETs) is investigated. We have found the highest mobilities in a class of symmetrically substituted PPVs with linear alkyl chains (e.g., R¹, R² = n‐C₁₁H₂₃, R³ = n‐C₁₈H₃₇). We have shown that the mobility of these PPVs can be improved significantly up to values of 10^–2 cm² V^–1 s^–1 by annealing at 110 °C. In addition, these devices display an excellent stability in air and dark conditions. No change in the electrical performance is observed, even after storage for thirty days in humid air.     

Preparation of ultrasmooth and defect-free 4H-SiC(0001) surfaces by elastic emission machining

In this work, elastic emission machining (EEM), which is a precise surface-preparation technique using chemical reactions between the surfaces of work and fine powder particles, is applied to the flattening 4H-SiC (0001) surface. Prepared surfaces are observed and characterized by optical interferometry, atomic force microscopy (AFM), and low-energy electron diffraction (LEED). The obtained images show that the processed surface has atomic-level flatness, and the subsurface damage and surface scratches of the preprocessed surface are almost entirely removed.     

PPV的合成和光致发光光谱

用Ｌｅｎｚ法和Ｂｕｒｎ法合成了ＰＰＶ,发现热转化温度是改变光致发光光谱的主要因素,随着热转化温度升高,谱峰发生红移、强度下降,谱峰形状很大;轻微的氧化、膜厚对发光强度有影响,对谱峰位置、形状影响很小;另在析不完全会Ｌｅｎｚ法合成的ＰＰＶ谱峰发生蓝移和形状变化。     

Shape stabilization and size equalization of InGaAs self-organized quantum dots

We report a simultaneous shape stabilization and size equalization after shape transformation of InGaAs self-organized quantum dots (QDs) formed via a fractional monolayer (ML) deposition technique. The density of QD increases rapidly from an initial value of 110±10/μm² (at a total deposition of 4 ML) to 270±30/μm² (at 5 ML) and saturates at a level of 240±20/μm² (at 10 ML). At an intermediate stage of 7 ML deposition, bimodal QD height (peaked at 8.5 nm and 14.5 nm) and aspect ratio (peaked at 0.18 and 0.26) distributions occur, confirming the QD shape transformation from a shallower to a steeper shape. The eventual convergence in lateral size, height and aspect ratio is the direct result of the simultaneous QD size equalization and shape stabilization. The QD size and shape evolution is also substantiated by the low temperature (4 K) photoluminescence (PL) data taken from samples with QDs capped by GaAs.