Confocal Raman Spectroscopy of α‐Sexithiophene: From Bulk Crystals to Field‐Effect Transistors

We report confocal micro‐Raman spectra of the organic semiconductor α‐sexithiophene (T6) on bulk crystals and on thin films grown on technologically relevant substrates and devices. We show that the two polymorphs, which are clearly identified by their lattice phonon spectra, may coexist as physical impurities of one inside the other in the same crystallite. Spatial distribution of the two phases is monitored by Raman phonon mapping of crystals grown upon different conditions. Raman microscopy has then been extended to T6 thin films grown on silicon oxide wafers. We identify the crystal phase in thin films whose thickness is just 18 nm. The most intense total‐symmetric Raman vibration is still detectable for a two‐monolayer thick film. Comparative analysis between micro‐Raman and AFM of T6 thin films grown on field effect transistors shows that electrode‐channel steps favour the nucleation and growth of T6 molecules on the substrate, at least below 50 nm.     

Highly Efficient and Thermally Stable Electro‐Optical Dendrimers for Photonics

After a brief review on electro‐optical (EO) polymers, the recent development of EO dendrimers is summarized. Both single‐ and multiple‐dendron‐modified nonlinear optical (NLO) chromophores in the guest–host polymer systems showed a very significant enhancement of poling efficiency (up to a three‐fold increase) due to the minimization of intermolecular electrostatic interactions among large dipole moment chromophores through the dendritic effect. Moreover, multiple NLO chromophore building blocks can also be placed into a dendrimer to construct a precise molecular architecture with a predetermined chemical composition. The site‐isolation effect, through the encapsulation of NLO moieties with dendrons, can greatly enhance the performance of EO materials. A very large EO coefficient (r₃₃ = 60 pm/V at 1.55 μm) and high temporal stability (85 °C for more than 1000 h) were achieved in a NLO dendrimer (see Figure) through the double‐end functionalization of a three‐dimensional phenyl‐tetracyanobutadienyl (Ph‐TCBD)‐containing NLO chromophore with thermally crosslinkable trifluorovinylether‐containing dendrons.     

Balancing Intermolecular and Molecule–Substrate Interactions in Supramolecular Assemblies

Self‐assembly of functional supra‐molecular nanostructures is among the most promising strategies for further development of organic electronics. However, a poor control of the interactions driving the assembling phenomena still hampers the tailored growth of designed structures. Here exploration of how non‐covalent molecule‐substrate interactions can be modified on a molecular level is described. For that, mixtures of DIP and F16CuPc, two molecules with donor and acceptor character, respectively are investigated. A detailed study of their structural and electronic properties is performed. In reference to the associated single‐component layers, the growth of binary layers results in films with strongly enhanced intermolecular interactions and consequently reduced molecule‐substrate interactions. This new insight into the interplay among the aforementioned interactions provides a novel strategy to balance the critical interactions in the assembly processes by the appropriate choice of molecular species in binary supra‐molecular assemblies, and thereby control the self‐assembly of functional organic nanostructures.     

Intermolecular Interactions of Perylene diimides in Photovoltaic Blends of Fluorene Copolymers: Disorder Effects on Photophysical Properties,Film Morphology and Device Efficiency

In the present work, we correlate the photophysical and photovoltaic properties with the respective film morphologies of three different blends made of the fluorene copolymers poly(9,9′‐dioctylfluorene‐co‐benzothiadiazole) (F8BT), poly[9,9′‐dioctylfluorene‐co‐N‐(4‐butylphenyl)diphenylamine] (TFB), and poly[9,9′‐dioctyfluorene‐co‐bis‐N,N′‐(4‐butylphenyl)‐bis‐N,N‐phenyl‐1,4‐phenylenediamine] (PFB) when blended with a perylene tetracarboxylic diimide (PDI) derivative. Additional photophysical studies in reference PDI blends of the electronically inert poly(styrene) matrix address the enhanced PDI intermolecular solid‐state interactions. We resolve the process of resonance energy transfer from excited polymer hosts to PDI and the process of photoinduced hole transfer from PDI to the polymer hosts. We deduce the efficiency of charge‐transfer PDI photoluminescence (PL) quenching and we discuss the power‐law PL kinetics seen in the as‐spun systems. Next we determine the dependence of the device external quantum efficiency (EQE) of these blends, in a range of annealing temperatures and PDI loadings. Differential scanning calorimetry enables precise selection of annealing temperatures. Optical microscopy shows that annealing enhances the order characteristics in the PDI aggregates in the F8BT:PDI system. In the case of the TFB:PDI and PFB:PDI blends, AFM studies suggest the formation of PDI‐rich domains on the film/air interface. The degree of order in the π–π stacking of the PDI monomers is inferred by the UV–Vis and PL spectra of the blends. The extent of order characteristics in PDI aggregates is correlated with the thermal properties of the hosts that control PDI molecular mobility upon annealing. The efficient dispersion of disrupted PDI crystallites is proposed to form appropriate percolation networks that favor balanced extraction of photogenerated carriers.     

Cover Picture: High Definition Digital Fabrication of Active Organic Devices by Molecular Jet Printing (Adv. Funct. Mater. 15/2007)

A new method for direct patterning of organic optoelectronic/electronic devices using a reconfigurable and scalable printing method is reported by Vladimir Bulovic and co‐workers on p. 2722. The printing technique is applied to the fabrication of high‐resolution printed organic light emitting devices (OLEDs) and organic field effect transistors (OFETs). Remarkably, the final print‐deposited films are evaporated onto the substrate (rather than solvent printed), giving high‐quality, solvent‐free, molecularly flat structures that match the performance of comparable high‐performance unpatterned films. We introduce a high resolution molecular jet (MoJet) printing technique for vacuum deposition of evaporated thin films and apply it to fabrication of 30 μm pixelated (800 ppi) molecular organic light emitting devices (OLEDs) based on aluminum tris(8‐hydroxyquinoline) (Alq₃) and fabrication of narrow channel (15 μm) organic field effect transistors (OFETs) with pentacene channel and silver contacts. Patterned printing of both organic and metal films is demonstrated, with the operating properties of MoJet‐printed OLEDs and OFETs shown to be comparable to the performance of devices fabricated by conventional evaporative deposition through a metal stencil. We show that the MoJet printing technique is reconfigurable for digital fabrication of arbitrary patterns with multiple material sets and high print accuracy (of better than 5 μm), and scalable to fabrication on large area substrates. Analogous to the concept of “drop‐on‐demand” in Inkjet printing technology, MoJet printing is a “flux‐on‐demand” process and we show it capable of fabricating multi‐layer stacked film structures, as needed for engineered organic devices.     

Nanocrystalline Electroplated Cu–Ni: Metallic Thin Films with Enhanced Mechanical Properties and Tunable Magnetic Behavior

Nanocrystalline 3 µm thick Cu_1–xNi_x (0.45 ≤ x ≤ 0.87) films are electrodeposited galvanostatically onto Cu/Ti/Si (100) substrates, from a citrate‐ and sulphate‐based bath containing sodium lauryl sulphate and saccharine as additives. The films exhibit large values of reduced Young's modulus (173 < E_r < 192 GPa) and hardness (6.4 < H < 8.2 GPa), both of which can be tailored by varying the alloy composition. The outstanding mechanical properties of these metallic films can be ascribed to their nanocrystalline nature—as evidenced by X‐ray diffraction, transmission electron microscopy, and atomic force microscopy—along with the occurrence of stacking faults and the concomitant formation of intragranular nanotwins during film growth. Due to their nanocrystalline character, these films also show very low surface roughness (root mean square deviation of around 2 nm). Furthermore, tunable magnetic properties, including a transition from paramagnetic to ferromagnetic behavior, are observed when the Ni percentage is increased. This combination of properties, together with the simplicity of the fabrication method, makes this system attractive for widespread technological applications, including hard metallic coatings or magnetic micro/nano‐electromechanical devices.     

GaAs/InGaAs应变异质结构临界厚度的Raman散射和PL谱分析

采用室温Raman散射和低温光致发光(PL)谱,对以TMG,固体As和固体In作为分子束源的MOMBE法生长的GaAs/In_xGa_(1-x)As(x=0.3)单层异质结构和多量子阱结构中InGaAs应变层的临界厚度进行了实验研究。由应变引起的Raman散射峰位移,以及PL谱峰位置与应变和无应变状态下一维有限深势阱跃迁能量计算结果的比较可见,在In组分含量x=0.3的情况下,临界厚度H_c≤5nm,小于能量平衡理论的结果,而与力学平衡模型的理论值相近。     

Si/Al2O3/ZnO:Al capacitor arrays formed in electrochemically etched porous Si by atomic layer deposition

High surface area Si/Al₂O₃/ZnO:Al capacitors were formed in electrochemically etched porous silicon. The Al₂O₃ dielectric and the ZnO:Al top electrode were deposited by atomic layer deposition in high aspect ratio porous Si. A single capacitor with a typical area of about 1 mm² consisted of about 10⁵ pores. Effective capacitance densities were between 2.0 and 2.5 μF/cm², i.e., approximately 30 times higher than for a planar capacitor prepared under identical conditions, illustrating the effect of the enhanced surface area in the porous structure.     

Temperature dependent study of basal plane stacking faults in Ag:ZnO nanorods by Raman and photoluminescence spectroscopy

We report the specific features of basal plane stacking faults (BSFs) in ZnO nanorods (NRs), studied by temperature dependent photoluminescence and Raman spectroscopy. At low temperature (4 K) the intense band of emission at 3.321 eV is attributed to the presence of BSFs defects and Ag as an acceptor dopant in ZnO. This specific peak red-shifts with the temperature increase, occupying the position 3.210 eV at RT. The nature of the emission is explained as exciton recombination of the electrons, confined in the homo-heterojunction QW, with the holes, localized near the Ag atoms close to SFs. Raman spectroscopy revealed that Ag:ZnO nanorods have slightly downshifted positions of the modes 330 cm⁻¹ and 440 cm⁻¹ by 4 cm⁻¹, which we explain as due to the presence of BSFs. It was also observed, that the longitudinal optical phonon mode A^LO, which is common polar mode for ZnO, was not detected by Raman spectroscopy in the samples with high BSFs density. This feature can be explained as due to existence of the bound charge induced by the BSFs in the NRs.     

Submicrometer‐Sized ZIF‐71 Filled Organophilic Membranes for Improved Bioethanol Recovery: Mechanistic Insights by Monte Carlo Simulation and FTIR Spectroscopy

Template‐free self‐assembly synthesis of nano‐sized metal‐organic frameworks (MOFs) is of particular interest in MOF research since organized nanostructures possessing distinctive properties are useful for many advanced applications. In this work, the facile room temperature synthesis of robust submicrometer‐sized ZIF‐71 crystals with different particle sizes (140, 290, or 430 nm), having a high permanent microporosity (S_BET = 827 cm² g^?1) and synthesis yield up to 80% based on Zn on a gram‐scale, is reported. These small ZIF‐71 particles are ideal filler for the fabrication of thinner and homogeneous polydimethylsiloxane (PDMS) based mixed matrix membranes (MMMs) with excellent filler dispersion and filler‐polymer adhesion at high loading up to 40 wt%, as confirmed by scanning electron microscopy. Pervaporation tests using these submicrometer‐sized ZIF‐71 filled MMMs show significant improvement for bioethanol recovery. Interesting phenomena of i) reversible ethanol‐ethanol hydrogen interaction in the ethanol liquid‐phase and ii) irreversible hydrogen interaction of ethanol and –Cl functional group in the α‐cages and octagonal prismatic cages of ZIF‐71 in ethanol vapor‐phase are discovered for the first time by a Fourier transform infrared spectroscopy (FTIR) study. In full agreement with molecular simulation results, these explain fundamentally the ZIF‐71 filled MMMs pervaporation performance.     

Color Tuning in Benzo[1,2,5]thiadiazole‐Based Small Molecules by Amino Conjugation/Deconjugation: Bright Red‐Light‐Emitting Diodes

Bipolar compounds (referred to in general as btza ) containing a benzo[1,2,5]thiadiazole core and peripheral diarylamines and/or 4‐tert‐butylphenyl moieties have been synthesized via palladium‐catalyzed cross‐coupling reactions of 4,7‐dibromobenzo[1,2,5]thiadiazole with appropriate stannyl compounds. These compounds are fluorescent and the emission color ranges from green to red. The fluorescence of the compounds originates from a charge‐transfer process and exhibits solvatochromism. These red‐light‐emitting materials are amorphous and devices of different configurations were fabricated: I) ITO/ btza /TPBI/Mg:Ag; II) ITO/ btza /Alq₃/Mg:Ag; III) ITO/ btza /Mg:Ag (where ITO = indium tin oxide, TPBI = 1,3,5‐tris(N‐phenylbezimidazol‐2‐yl)benzene, and Alq₃ = tris(8‐hydroxyquinoline)aluminum). The performance of some of the red‐light‐emitting devices appears to be very promising.     

Cover Picture: A Route to Self‐Organized Honeycomb Microstructured Polystyrene Films and Their Chemical Characterization by ToF‐SIMS Imaging (Adv. Funct. Mater. 7/2007)

The cover shows a composition of different characterization images of an auto‐organized polystyrene film obtained through breath‐figure imprinting, as reported by Sami Yunus and co‐workers on p. 1079. Water‐droplet condensation, represented as a synthetic perspective image (top), is responsible for ordered microstructuring during film formation. The following perspectives are taken from SEM and from three negative ToF‐SIMS images that allow deduction of the surface chemical composition. The background is an SEM picture of a polydimethylsiloxane molding of the self‐organized film. A new type of polymer compound that allows the formation of highly ordered microstructured films by casting from a volatile solvent in the presence of humidity, and its characterization by ToF‐SIMS (time‐of‐flight secondary‐ion mass spectrometry) are presented. A honeycomb structure is obtained by activation of 2,2,6,6‐tetramethyl‐1‐piperidinyloxyl (TEMPO)‐terminated polystyrene (PS) with p‐toluenesulfonic acid (PTSA). The mechanism of this activation reaction, leading to a more polar PS termination, is deduced from simple experiments and supported by ToF‐SIMS characterization. Positive and negative ToF‐SIMS imaging allows different chemical regions correlating to the film morphology to be distinguished. This new, straightforward activation process, together with ToF‐SIMS chemical imaging, provides a better understanding of the phenomena underlying the formation of these films by directly linking the role of polar terminations to the microscale self‐organization. This new method, transposable to other organic acids, suggests interesting new perspectives in the field of self‐organized chemical and topographical patterning.     

Luminescence and Amplified Stimulated Emission in CdSe–ZnS‐Nanocrystal‐Doped TiO2 and ZrO2 Waveguides

A reproducible route for the preparation of high‐quality CdSe–ZnS‐doped titania and zirconia waveguides is presented. The optical properties of the resultant composite materials are found to be sensitive to the semiconducting properties of the host matrix. Titania‐based composites are seen to be inherently photounstable because of photoelectron injection into the bulk matrix and subsequent nanocrystal (NC) oxidation. In comparison, zirconia composites are significantly more robust with high photoluminescence (PL) retained for annealing temperatures up to 300 °C. Both titania and zirconia composite waveguides exhibit amplified stimulated emission (ASE); however only zirconia‐based waveguides exhibit long‐term photostability (loss of less than 30 % ASE intensity after more than 40 min continuous excitation). We conclude that the low electron affinity of zirconia and its inherent high refractive index makes it an ideal candidate for NC‐based optical waveguides.     

(Pb1–xCdx)S Nanoparticles Embedded in a Conjugated Organic Matrix,as Studied by Photoluminescence and Light‐Induced X‐ray Photoelectron Spectroscopy

Elliptically shaped (Pb_1–xCd_x)S nanoparticles (NPs) of average size 2.3 × 2.9 nm (minor axis × major axis) have been prepared via reaction of a solid [oligo(p‐phenylene‐ethynylene) dicarboxylate]Pb_0.9Cd_0.1 salt matrix, with gaseous H₂S. A significantly long emission lifetime, with multi‐exponential behavior, is detected in time‐resolved photoluminescence measurements, substantially different from the decay patterns of pure PbS and CdS NPs within the same organic matrix. Evidence for the co‐existence of Cd and Pb within the same particle is provided by light‐induced X‐ray photoelectron spectroscopy.     

Optical and electrical properties of epitaxial (Mg,Cd)xZn1−xO, ZnO, and ZnO:(Ga,Al) thin films on c-plane sapphire grown by pulsed laser deposition

A consistent set of epitaxial, n-type conducting ZnO thin films, nominally undoped, doped with Ga or Al, or alloyed with Mg or Cd, was grown by pulsed laser deposition (PLD) on single-crystalline c-plane sapphire (0 0 0 1) substrates, and characterized by Hall measurement, and UV/VIS optical transmission spectroscopy.The optical band gap of undoped ZnO films at nearly 3.28 eV was shifted by alloying with Mg up to 4.5 eV and by alloying with Cd down to 3.18 eV, dependent on the alloy composition. In addition, highly doped ZnO:Al films show a blue-shifted optical absorption edge due to filling of electronic states in the conduction band.The Hall transport data of the PLD (Mg,Zn,Cd)O:(Ga,Al) thin films span a carrier concentration range of six orders of magnitude from 3 × 10¹⁴ to 3 × 10²⁰ cm⁻³, which corresponds to a resistivity from 5 × 10⁻⁴ to 3 × 10³ Ω cm. Structurally optimized, nominally undoped ZnO films grown with ZnO nucleation and top layer reached an electron mobility of 155 cm²/V s (300 K), which is among the largest values reported for heteroepitaxial ZnO thin films so far.Finally, we succeeded in combining the low resistivity of ZnO:Ga and the band gap shift of MgZnO in MgZnO:Ga thin films. This results demonstrate the unique tunability of the optical and electrical properties of the ZnO-based wide-band gap material for future electronic devices.     

Determination of Crystal Axes in Semimetallic T′‐MoTe2 by Polarized Raman Spectroscopy

Distorted octahedral T′ phase of MoTe₂ has recently attracted significant interest due to its predicted topological states and novel charge transport properties. Here, we report a nondestructive method for determining the crystal orientation of few‐layer T′‐MoTe₂ flakes by polarized Raman spectroscopy. The experimentally observed Raman modes are assigned to eigenmodes of vibrations predicted by density functional theory calculations. Polarized Raman measurements reveal four distinct types of angle‐dependent intensity variations. From group theory, it can be deduced that the intensity of the B_g mode reaches a maximum in the configuration when the polarization vector of the incident light is either parallel or orthogonal to the metal–metal zigzag chain direction. The intensity variation of the B_g mode cannot be used to unambiguously determine the crystal orientation. Using electron diffraction analysis, it is demonstrated that the intensity of the A_g mode at around 162 cm^?1 reaches a maximum when the polarization vector of the incident light is parallel to the metal–metal chain direction in the configuration. Furthermore, a simple method is proposed for identifying crystal orientation in nonpolarized Raman spectroscopy.