Electronic Properties and Structure of Assemblies of CdSe Nanocrystal Quantum Dots and Ru‐Polypyridine Complexes Probed by Steady State and Time‐Resolved Photoluminescence

Chemical and electronic interactions between CdSe nanocrystal quantum dots (NQDs) and Ru‐polypyridine complexes are studied in solution. It is shown that photoluminescence (PL) can be used to effectively monitor the formation of NQD‐complex assemblies in real time. It is also shown that with the aid of Langmuir isotherm modeling, the PL studies can be used to quantitatively characterize the composition of the assemblies and the strength of electronic interactions between their components. The approach demonstrated here is general and can be applied to other systems that combine semiconductor NQDs and appropriately functionalized organometallic or organic molecules interacting with NQDs via energy transfer, charge transfer, or other mechanisms leading to quenching of NQD emission.     

Structural and Electronic Properties of the First Monolayers of Spin‐Cast Poly(fluorene)‐Based Conjugated‐ Polymer Films

This study is an extended investigation on the formation of the first few monolayers of conjugated poly(fluorene)‐based polymer films prepared from solution on defined polar and nonpolar surfaces. In particular, a symmetrical A–B–A triblock copolymer consisting of poly(2‐alkylaniline) as A blocks and poly(9,9‐dialkylfluorene) as B blocks and a poly(9,9‐dialkylfluorene) homopolymer is used for this study. The dependence on drying conditions by means of solvent selection, the influence of a subsequent heat treatment, and the influence of the substrate polarity are investigated for ultrathin films as well as the transition from the first monolayers to the bulk polymer. The study is performed with optical absorption and photoluminescence spectroscopy, and atomic force microscopy to obtain complementary information of optical properties and morphological details. We find that ultrathin films (ca. 1–2 nm) prepared on mica from various solvents form highly defined, flat monolayers at the interface without lateral regularities indicating a dipole–dipole interaction between conjugated‐polymer segments and mica surface dipoles. This is further confirmed by bathochromic photoluminescence shifts observed for the ultrathin layers compared to the bulk polymer. Complementary experiments on nonpolar surfaces, highly oriented pyrolytic graphite (HOPG), show a total absence of defined flat films supporting the assumption of a dipole–dipole assisted formation on mica. For increased film thickness on mica (5 nm and more) the homopolymer does not form any regular structures or ordered layers on top of the monolayer. In contrast, the triblock copolymer, shorter in length, revealed a tendency to form a less‐defined layer‐type growth (3–3.5 nm thick) above the monolayer that was of higher order for higher‐boiling‐point solvents, indicating that the polymers are found in a different conformation. Moreover, one finds that some solvents that show partial immiscibility with the polymer strongly alter the formation of the film. The observations made for the two different types of polymers allow for an assignment of film‐formation driving forces to individual polymer segments and allow for the formulation of a growth model that explains the observed results and indicates the importance of appropriate substrate selection for organic electronic/optoelectronic devices.     

GaN基LED与Si键合技术的研究

采用金属键合技术结合激光剥离技术将GaN基LED从蓝宝石衬底成功转移到Si衬底上。利用X射线光电子谱(XPS)研究不同阻挡层对Au向GaN扩散所起的阻挡作用,确定键合所需的金属过渡层。利用多层金属过渡层,在真空、温度400℃和加压300 N下实现GaN基LED和Si的键合,通过激光剥离技术将蓝宝石衬底从键合结构上剥离下来,形成GaN基LED/金属层/Si结构。用金相显微镜及原子力显微镜(AFM)观察结构的表面形貌,测得表面粗糙度(RMS)为12.1 nm。X射线衍射(XRD)和Raman测试结果表明,衬底转移后,GaN基LED的结构及其晶体质量没有发生明显变化,而且GaN与蓝宝石衬底间的压应力得到了释放,使得Si衬底上GaN基LED的电致发光(EL)波长发生红移现象。     

Hole transfer from PbS nanocrystal quantum dots to polymers and efficient hybrid solar cells utilizing infrared photons

Polymer/inorganic-nanocrystals bulk heterojunction solar cells, where inorganic semiconductor nanocrystals such as CdSe, CdS, CdTe, ZnO, TiO₂, and silicon, replace the fullerene molecules as the electron acceptors, typically exhibit a power conversion efficiency (PCE) below 3% even after tremendous engineering efforts to optimize the nanocrystal size, shape, and nanoscale morphology. One promising feature of polymer hybrid solar cells is the ability to sensitize conjugated polymers, which on their own absorb only in the visible part of solar spectrum, into the infrared spectral range using infrared-active lead salt nanocrystal quantum dots (NQDs). Here we observed for the first time hole transfer from PbS NQDs to polymers as evidenced by the quenching of the PbS photoluminescence (PL), a sign of the presence of charge separating type II heterojunction. The type II band-offset at the NQD/polymer heterojunction enables efficient hole extraction from NQDs and leads to a record PCE of 3.80%, realized in a planar junction configuration under simulated air mass 1.5 global (AM 1.5G) irradiation of 100 mW/cm². The photocurrent has an extended spectral range spanning from the ultraviolet (UV) to the infrared (IR). Contributions from the polymer and PbS to the photocurrent were identified. Infrared photons (>700 nm) contribute about 30% of the photocurrent and yield a high external quantum efficiency (EQE) of 20% at 1050 nm.     

微腔结构顶发射有机白光器件

结合微腔效应,通过调节不同发光层的厚度制作了顶发射有机白光器件.器件结构为Si/Ag/Ag2O/m-MTDATA/NPB/DPVBi/DCJTB:Alq3/Alq3/LiF/Al/Ag,其中DPVBi,DCJTB与Alq3的掺杂层分别作为蓝光和红光发光层,在选定490 nm的谐振波长时,通过调节DPVBi和掺杂层的厚度来实现对器件发光色度的调节.当DPVBi厚度为1 nm,电压为9 V时,器件的色坐标为(0.33,0.34),非常接近白光等能点.此项工作为利用微腔效应制作高效率高亮度顶发射白光器件奠定了基础.     

STM Lithography in an Organic Self‐Assembled Monolayer

We describe the suitability of ultra‐high vacuum scanning tunneling microscopy (UHV‐STM) based nanolithography by using highly ordered monomolecular organic films, called self‐assembled monolayers (SAMs), as ultrathin resists. Organothiol‐type SAMs such as hexadecanethiol (SH–(CH₂)₁₅–CH₃) and N‐biphenylthiol (SH–(C₆H₆)₂–NO₂) monolayers have been prepared by immersion on gold films and Au(111) single crystals. Organosilane‐type SAMs such as octadecyltrichlorosilane (SiCl₃–(CH₂)₁₇–CH₃) monolayers have been prepared on hydroxylated Si(100) surfaces as well as hydroxylated chromium film surfaces. Dense line patterns have been written by UHV‐STM in constant current mode for various tunneling parameters (gap voltage, tunneling current, scan speed, and orientation) and transferred into the underlying substrate by wet etch techniques. The etched structures have been analyzed by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM). Best resolution has been achieved without etch transfer for a 20 nm × 20 nm square written in hexadecanethiol/Au(111) with an edge definition of about 5 nm. Etch transfer of the STM nanopatterns in Au films resulted in 55 nm dense line patterns (15 nm deep) mainly broadened by the isotropic etch characteristic, while 35 nm wide and 30 nm deep dense line patterns written in octadecyltrichlorosilane/Si(100) and anisotropically etched into Si(100) could be achieved.     

Band Structure and Photoelectric Characterization of GeSe Monolayers

Germanium selenide monolayer is promising in photoelectric applications for its natural p‐type semiconductor and complicated band structures. Basic experimental investigations of few‐to‐monolayer germanium selenide are still absent; major scientific challenge is to develop of techniques for controllably thinned monolayers. In this study laser thinned monolayer germanium selenide on SiO₂/Si substrates is demonstrated. A broad photoluminescence spectrum with eight continues peaks is observed from visible to infrared wavebands centered at ≈589, 655, 737, 830, 1034, 1178, 1314, and 1456 nm, respectively. First‐principle calculations based on density functional theory illuminate the band structures of few‐to‐monolayer germanium selenide. Photoluminescence investigation combined with first‐principle calculations indicates that the indirect to direct bandgap transition happens at few layers of N = 3. Current–voltage and photoresponse characteristics of monolayer based devices show 3.3 times the photosensitivity and much faster falling edges compared with those of the pristine nanosheet based devices. The present results provide useful insight into deep understanding of thickness dependent performances of germanium selenide monocrystalline.     

LB技术制备金纳米粒子单层膜

利用LB技术在Si(100)基片上制作了金纳米粒子单层膜,采用扫描电子显微镜(SEM)和原子力显微镜(AFM)对单层膜的表面形貌进行表征。实验结果表明,将表面压控制在22mN/m～26mN/m进行拉膜,可以得到大面积金纳米粒子的均匀致密单层膜。研究结果对于采用金纳米粒子单层膜构建DNA传感器和单电子器件方面具有重要的应用价值。     

Electronic Contact Deposition onto Organic Molecular Monolayers: Can We Detect Metal Penetration?

Using a semiconductor as the substrate to a molecular organic layer, penetration of metal contacts can be clearly identified by the study of electronic charge transport through the layer. A series of monolayers of saturated hydrocarbon molecules with varying lengths is assembled on Si or GaAs and the junctions resulting after further electronic contact is made by liquid Hg, indirect metal evaporation, and a “ready‐made” metal pad are measured. In contrast to tunneling characteristics, which are ambiguous regarding contact penetration, the semiconductor surface barrier is very sensitive to any direct contact with a metal. With the organic monolayer intact, a metal–insulator–semiconductor (MIS) structure results. If metal penetrated the monolayer, the junction behaves as a metal–semiconductor (MS) structure. By comparing a molecule‐free interface (MS junction) with a molecularly modified one (presumably MIS), possible metal penetration is identified. The major indicators are the semiconductor electronic transport barrier height, extracted from the junction transport characteristics, and the photovoltage. The approach does not require a series of different monolayers and data analysis is quite straightforward, helping to identify non‐invasive ways to make electronic contact to soft matter.     

Biocatalytic Nanoscale Coatings Through Biomimetic Layer‐by‐Layer Mineralization

Recent insight into the molecular mechanisms of biological mineral formation (biomineralization) has enabled biomimetic approaches for the synthesis of functional organic‐inorganic hybrid materials under mild reaction conditions. Here we describe a novel method for enzyme immobilization in thin (nanoscale) conformal mineral coatings using biomimetic layer‐by‐layer (LbL) mineralization. The method utilizes a multifunctional molecule comprised of a naturally‐occurring peptide, protamine (PA), covalently bound to the redox enzyme Glucose oxidase (GOx). PA mimics the mineralizing properties of biomolecules involved in silica biomineralization in diatoms, and its covalent attachment to GOx does not interfere with the catalytic activity. Highly efficient and stable incorporation of this modified enzyme (GOx‐PA) into nanoscale layers (～5–7 nm thickness) of Ti‐O and Si‐O is accomplished during protamine‐enabled LbL mineralization on silica spheres. Depending on the layer location of the enzyme and the type of mineral (silica or titania) within which the enzyme is incorporated, the resulting multilayer biocatalytic hybrid materials exhibit between 20–100% of the activity of the free enzyme in solution. Analyses of kinetic properties (V_max, K_M) of the immobilized enzyme, coupled with characterization of physical properties of the mineral‐bearing layers (thickness, porosity, pore size distribution), indicates that the catalytic activities of the synthesized hybrid nanoscale coatings are largely determined by substrate diffusion rather than enzyme functionality. The GOx‐PA immobilized in these nanoscale layers is substantially stabilized against heat‐induced denaturation and largely protected from proteolytic attack. The method for enzyme immobilization described here enables, for the first time, the high yield immobilization and stabilization of enzymes within continuous, conformal, and nanoscale coatings through biomimetic LbL mineralization. This approach will likely be applicable to a wide variety of surfaces and functional biomolecules. The ability to synthesize thin (nanoscale) conformal enzyme‐loaded layers is of interest for numerous applications, including enzyme‐based biofuel cells and biosensors.     

Simultaneous and Dual Emissive Imaging by Micro‐Contact Printing on the Surface of Electrostatically Assembled Water‐Soluble Poly(p‐phenylene) Using FRET

Poly{[2,5‐bis(3‐sulfonatobutoxy)‐1,4‐phenylene sodium salt]‐alt‐(1,4‐phenylene)}, which is an anionically charged, water‐soluble poly(para‐phenylene) derivative with aldehyde groups at both chain ends, is prepared via the Suzuki coupling reaction in order to develop a FRET energy donor, while simultaneously dual‐fluorescence‐patterning the protein. Regardless of the end‐capping, the synthesized polymer exhibits a good solubility in water with an absorption maximum at 338 nm and a photoluminescence maximum at 417 nm, similar to those of the the end‐capped polymer. The emission spectrum of the polymer overlaps the absorption spectrum of fluorescein, and therefore, the polymer can be used as an energy donor with fluorescein as the energy acceptor in the FRET mechanism. This polymer design not only takes advantage of the introduction of biotin at both chain ends (through a reaction with the aldehyde end groups) to realize the facile interaction with streptavidin, but also brings into play the electrostatic features of the anionic sulfonate groups to fabricate an electrostatic self‐assembly with polycation for the pattern substrate. The micropattern of fluorescein‐labeled streptavidin is fabricated on the polymer‐coated substrate through micro‐contact printing using a polydimethylsiloxane mold. As a result, the polymer substrate exhibits a dual fluorescence micropattern, which results from the blue emission color from the energy donor and the FRET‐amplified green emission from the energy acceptor. The high‐resolution patterning is carried out for the application of multiplexing by simultaneously imaging the patterned green‐emitting fluorescein by FRET and the surrounding blue‐emitting polymer according to an optical detection scheme.     

一种新型全方位反射AlGaInP薄膜发光二极管

提出了一种新型全方位反射A1GalnP LED结构和制作工艺.GaAs外延片与含导电孔的SiO2,Au形成全方位反射镜后,银浆键合在Si支架上,去除GaAs衬底,制作薄AuGeNi电极,粗化,生长ITO,制作厚AuGeNi电极,合金则形成ODR薄膜LED结构.300μm×300μm管芯裸装在TO-18金属管座上,在20mA的电流驱动下,测得电压为2.2V,光强达到195mcd,光功率达到3.78mW,比常规吸收衬底LED提高3.6倍.     

Stretchable Active Matrix Inorganic Light‐Emitting Diode Display Enabled by Overlay‐Aligned Roll‐Transfer Printing

An active matrix‐type stretchable display is realized by overlay‐aligned transfer of inorganic light‐emitting diode (LED) and single‐crystal Si thin film transistor (TFT) with roll processes. The roll‐based transfer enables integration of heterogeneous thin film devices on a rubber substrate while preserving excellent electrical and optical properties of these devices, comparable to their bulk properties. The electron mobility of the integrated Si‐TFT is over 700 cm² V^?1 s^?1, and this is attributed to the good interface between the Si channel and the thermally grown SiO₂ insulator. The light emission properties of the LED are of wafer quality. The resulting display stably operates under tensile strains up to 40%, over 200 cycles, demonstrating the potential of stretchable displays based on inorganic materials.     

Microdroplet and Atomic Force Microscopy Probe Assisted Formation of Acidic Thin Layers for Silicon Nanostructuring

In this work, an aqueous acidic thin‐layer‐based strategy for fabricating nanostructures on silicon by using atomic force microscopy (AFM) nanolithography is presented. The approach involves the formation of microscale droplets via dilute hydrofluoride (DHF) etching, the conversion of the droplets to acidic thin layers by AFM‐probe scanning, and subsequent lithographic operations using a biased probe in the aqueous layers. By varying the concentration of the acidic DHF layers, the thin layers can facilitate the creation of both positive and negative patterns, such as oxide dots and Si pores, through anodic oxidation and dissolution. In particular, the anodic oxidation in the acidic media is associated with the field‐enhanced nonequilibrium dissociation of the weak electrolyte. The Si pore structure formation is related to the field‐assisted dissolution of anodic oxides and the Si substrate. The acidic‐layer‐based technique allows switching between different lithographic modes by changing the acidity of the DHF layers, and is complementary to bulk solution‐based and local meniscus‐based approaches in AFM nanolithography. In principle, this method can also be extended to other materials that have similar reactions with DHF.     

High‐Sensitivity p–n Junction Photodiodes Based on PbS Nanocrystal Quantum Dots

Chemically synthesized nanocrystal quantum dots (NQDs) are promising materials for applications in solution‐processable optoelectronic devices such as light emitting diodes, photodetectors, and solar cells. Here, we fabricate and study two types of p‐n junction photodiodes in which the photoactive p‐layer is made from PbS NQDs while the transparent n‐layer is fabricated from wide bandgap oxides (ZnO or TiO₂). By using a p–n junction architecture we are able to significantly reduce the dark current compared to earlier Schottky junction devices without reducing external quantum efficiency (EQE), which reaches values of up to ～80%. The use of this device architecture also allows us to significantly reduce noise and obtain high detectivity (>10¹² cm Hz^1/2 W^?1) extending to the near infrared past 1 μm. We observe that the spectral shape of the photoresponse exhibits a significant dependence on applied bias, and specifically, the EQE sharply increases around 500–600 nm at reverse biases greater than 1 V. We attribute this behavior to a “turn‐on” of an additional contribution to the photocurrent due to electrons excited to the conduction band from the occupied mid‐gap states.     

Photoaddressable Alignment Layers for Fluorescent Polymers in Polarized Electroluminescence Devices

Liquid‐crystalline (LC) polyfluorenes have been successfully aligned on photoaddressable polymers (PAPs). This is the first example of the alignment of a LC main chain polymer on a photoaligned layer. The degree of molecular alignment in the fluorescent polyfluorene layer on top of an ultra‐thin PAP layer is shown to depend strongly on the chemical nature of the PAP. Good alignment with dichroic ratios of more than 10 was only achieved with PAPs containing liquid‐crystalline side chains. Patterning with laterally structured alignment was realized in several ways, utilizing reorientation with orthogonally polarized light. Thin PAP layers have further been utilized as hole‐conducting alignment layers in polymer light‐emitting diodes (LEDs) with polarized emission. In order to facilitate hole transport through the alignment layer, different concentrations of a hole‐transporting molecule (HTM) have been mixed into the PAP layer. These hole‐conducting alignment layers retained their aligning abilities even at HTM concentrations of 20 wt.‐%. LEDs with photometric polarization ratios in emission of up to 14 at a brightness of up to 200 cd/m² and an efficiency of 0.3 cd/A could be realized.     

Structural Determinants of the Sign of the Pyroelectric Effect in Quasi‐Amorphous SrTiO3 Films

The magnitude and direction of the permanent electric polarization in the non‐crystalline, polar phase (termed quasi‐amorphous) of SrTiO₃ in Si\SiO₂\Me\SrTiO₃\Me, (Me = Cr or W), Si\SrRuO₃\SrTiO₃, and Si\SrTiO₃ layered structures were investigated. Three potential sources of the polarization which appears after the material is pulled through a temperature gradient were considered: a) contact potential difference; b) a flexoelectric effect due to a strain gradient caused by substrate curvature; and c) a flexoelectric effect due to the thermally induced strain gradient that develops while pulling through the steep temperature gradient. Measurements show that options a) and b) can be eliminated from consideration. In most cases studied in this (Si\SrTiO₃, Si\SiO₂\Me\SrTiO₃\Me, M = Cr or W) and previous works (Si\BaTiO₃, Si\BaZrO₃), the top surface of the quasi‐amorphous phase acquires a negative charge upon heating. However, in Si\SrRuO₃\SrTiO₃ structures the top surface acquires a positive charge upon heating. On the basis of the difference in the measured expansion of the upper and lower surfaces of the SrTiO₃ layer in the presence and absence of SrRuO₃, we contend that the magnitude and direction of the pyroelectric effect are determined by the out‐of‐plane gradient of the in‐plane strain in the SrTiO₃ layer while pulling through the temperature gradient.     

Bright Blue Solution Processed Triple‐Layer Polymer Light‐Emitting Diodes Realized by Thermal Layer Stabilization and Orthogonal Solvents

The realization of fully solution processed multilayer polymer light‐emitting diodes (PLEDs) constitutes the pivotal point to push PLED technology to its full potential. Herein, a fully solution processed triple‐layer PLED realized by combining two different deposition strategies is presented. The approach allows a successive deposition of more than two polymeric layers without extensively redissolving already present layers. For that purpose, a poly(9,9‐dioctyl‐fluorene‐co‐N‐(4‐butylphenyl)‐diphenylamine) (TFB) layer is stabilized by a hard‐bake process as hole transport layer on top of poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). As emitting layer, a deep blue emitting pyrene‐triphenylamine copolymer is deposited from toluene solution. To complete the device assembly 9,9‐bis(3‐(5′,6′‐bis(4‐(polyethylene glycol)phenyl)‐[1,1′:4′,1″‐terphenyl]‐2′‐yl)propyl)‐9′,9′‐dioctyl‐2,7‐polyfluorene (PEGPF), a novel polyfluorene‐type polymer with polar sidechains, which acts as the electron transport layer, is deposited from methanol in an orthogonal solvent approach. Atomic force microscopy verifies that all deposited layers stay perfectly intact with respect to morphology and layer thickness upon multiple solvent treatments. Photoelectron spectroscopy reveals that the offsets of the respective frontier energy levels at the individual polymer interfaces lead to a charge carrier confinement in the emitting layer, thus enhancing the exciton formation probability in the device stack. The solution processed PLED‐stack exhibits bright blue light emission with a maximum luminance of 16 540 cd m^?2 and a maximum device efficiency of 1.42 cd A^?1, which denotes a five‐fold increase compared to corresponding single‐layer devices and demonstrates the potential of the presented concept.     

Facile Fabrication of High‐Density Sub‐1‐nm Gaps from Au Nanoparticle Monolayers as Reproducible SERS Substrates

The fabrication of ultrasmall nanogaps (sub‐1 nm) with high density is of significant interest and importance in physics, chemistry, life science, materials science, surface science, nanotechnology, and environmental engineering. However, it remains a challenge to generate uncovered and clean sub‐1‐nm gaps with high density and uniform reproducibility. Here, a facile and low‐cost approach is demonstrated for the fabrication of high‐density sub‐1‐nm gaps from Au nanoparticle monolayers as reproducible surface‐enhanced Raman scattering (SERS) substrates. Au nanoparticles with larger diameters possess lower surface charge, thus the obtained large‐area nanoparticle monolayer generates a high‐density of sub‐1‐nm gaps. In addition, a remarkable SERS performance with a 10¹¹ magnitude for the Raman enhancement is achieved for 120 nm Au nanoparticle monolayers due to the dramatic increase in the electromagnetic field enhancement when the obtained gap is smaller than 0.5 nm. The Au nanoparticle monolayer is also transferred onto a stretchable PDMS substrate and the structural stability and reproducibility of the high‐density sub‐1‐nm gaps in Au monolayer films are illustrated. The resultant Au nanoparticle monolayer substrates with an increasing particle diameter exhibit tunable plasmonic properties, which control the plasmon‐enhanced photocatalytic efficiency for the dimerization of p‐aminothiophenol. The findings reported here offer a new opportunity for expanding the SERS application.     

Efficient Blue Perovskite Light‐Emitting Diodes Boosted by 2D/3D Energy Cascade Channels

Lead halide perovskite, as an emerging semiconductor, provides a fire‐new opportunity for high‐definition display and solid‐state lighting. Earthshaking improvements are implemented in green, red, and near‐infrared perovskite light‐emitting diodes (PeLEDs). However, blue PeLEDs are still far behind in performance, which restricts the development of PeLEDs in practical applications. Herein, a facile energy cascade channel strategy via one‐step self‐organized and controllable 2D/3D perovskite preparation by introducing guanidine hydrobromide (GABr) is developed that greatly improves the efficiency of blue PeLEDs. The 2D/3D perovskite structure boosts the energy cascade to induce energy transfer from the wide into the narrow bandgap domains and inhibit free charge diffusion, which increases the density of electrons and holes, and enhances the radiative recombination. Profiting from this energy cascade channels, the external quantum efficiency of blue PeLEDs, emitting at 492 nm, is considerably enhanced from 1.5% of initial blue device to 8.2%. In addition, device operating stability under ambient conditions is also improved by 2.6‐fold. The one‐step self‐organized 2D/3D hybrid perovskites induced by GABr pave a new and simple route toward high‐performance blue emission PeLEDs.