Delayed ignition of autocatalytic combustion precursors: low-temperature nanomaterial binder approach to electronically functional oxide films

Delayed ignition of combustion synthesis precursors can significantly lower metal oxide film formation temperatures. From bulk In(2)O(3) precursor analysis, it is shown here that ignition temperatures can be lowered by as much as 150 °C. Thus, heat generation from ~60 nm thick In(2)O(3) films is sufficient to form crystalline In(2)O(3) films at 150 °C. Furthermore, we show that the low processing temperatures of sufficiently thick combustion precursor films can be applied to the synthesis of metal oxide nanocomposite films from nanomaterials overcoated/impregnated with the appropriate combustion precursor. The resulting, electrically well-connected nanocomposites exhibit significant enhancements in charge-transport properties vs conventionally processed oxide films while maintaining desirable intrinsic electronic properties. For example, while ZnO nanorod-based thin-film transistors exhibit an electron mobility of 10(-3)-10(-2) cm(2) V(-1) s(-1), encasing these nanorods within a ZnO combustion precursor-derived matrix enhances the electron mobility to 0.2 cm(2) V(-1) s(-1). Using commercially available ITO nanoparticles, the intrinsically high carrier concentration is preserved during nanocomposite film synthesis, and an ITO nanocomposite film processed at 150 °C exhibits a conductivity of ~10 S cm(-1) without post-reductive processing.     

Preparation of polyimide/BaTiO3‐nanocrystal hybrid thin films and their dielectric property

High dielectric constant is highly desirable in capacitors and memory devices. In this work, oleic acid (OA)‐capped BaTiO₃ nanocrystals were synthesized by a two‐phase approach. Polyimide (PI)/BaTiO₃‐nanocrystal composite thin films with high dielectric constant have been successfully fabricated. The morphologies and dielectric properties of the hybrid films were exploited. The results showed that BaTiO₃ nanocrystals can be uniformly dispersed in the PI thin films owing to the surface modification of OA‐capped BaTiO₃ nanocrystals. It was found that the dielectric constant of composite film varies with the volume fraction of BaTiO₃ nanocrystals and sintering temperatures and reaches a maximum value of 44.1, which is around 13 times higher than that of pristine PI thin film (3.2). These results demonstrated that PI/BaTiO₃‐nanocrystal composite films have considerable application potential in microelectronic fields.     

Fabrication and electron transport properties of epitaxial films of electron-doped 12CaO·7Al2O3 and 12SrO·7Al2O3

Epitaxial growth and electron doping of 12CaO·7Al₂O₃ (C12A7) and 12SrO·7Al₂O₃ (S12A7) are reported. The C12A7 films were prepared on Y₃Al₅O₁₂ (YAG) single-crystal substrates by pulsed laser deposition at room temperature and subsequent thermal crystallization. X-ray diffraction patterns revealed the films were grown epitaxially with the orientation relationship of (001)[100] C12A7 || (001)[100] YAG. For S12A7, pseudo-homoepitaxial growth was attained on the C12A7 epitaxial layer. Upon electron doping, metallic conduction was achieved in the C12A7 film and the S12A7/C12A7 double-layered films. Analyses of optical absorption spectra for the S12A7/C12A7 films provided the densities of free electrons in each layer separately. Hall measurements exhibited larger electron mobility in the S12A7/C12A7 film than those in C12A7 and S12A7 films, suggesting free electrons may be accumulated at the S12A7/C12A7 interface due presumably to a discontinuity of the cage conduction bands.     

Effect of the phase states of self-assembled monolayers on pentacene growth and thin-film transistor characteristics

To investigate the effects of the phase state (ordered or disordered) of self-assembled monolayers (SAMs) on the growth mode of pentacene films and the performance of organic thin-film transistors (OTFTs), we deposited pentacene molecules on SAMs of octadecyltrichlorosilane (ODTS) with different alkyl-chain orientations at various substrate temperatures (30, 60, and 90 degrees C). We found that the SAM phase state played an important role in both cases. Pentacene films grown on relatively highly ordered SAMs were found to have a higher crystallinity and a better interconnectivity between the pentacene domains, which directly serves to enhance the field-effect mobility, than those grown on disordered SAMs. Furthermore, the differences in crystallinity and field-effect mobility between pentacene films grown on ordered and disordered substrates increased with increasing substrate temperature. These results can be possibly explained by (1) a quasi-epitaxy growth of the pentacene film on the ordered ODTS monolayer and (2) the temperature-dependent alkyl chain mobility of the ODTS monolayers.     

Toward dynamic control over TiO2 nanocrystal monolayer-by-monolayer film formation by electrophoretic deposition in nonpolar solvents

The controlled electrophoretic deposition of monolayers and ultrathin films of 4.0 nm TiO(2) nanocrystals from stable, nonpolar solvent-based suspensions is reported. Stable suspensions were prepared in hexane, and the electrophoretic mobility of the nanocrystals was enhanced by a combination of a liquid-liquid extraction followed by mechanical surfactant removal by high-speed centrifugation. The controlled evolution of the density of TiO(2) nanocrystal monolayers was studied by transmission electron microscopy and optical transmittance spectroscopy. Ultrathin films were assembled while maintaining monolayer-by-monolayer growth and uniform density of the film. A time-dependent, equivalent circuit model has been proposed to characterize the electrophoretic current that was recorded during our experiments. Further, we demonstrate that the proposed model, coupled with the mobility, provides a means to estimate the deposition rate and, hence, the time necessary to fabricate a submonolayer, a monolayer, and multilayers of nanocrystals.     

Hydronium ion motion in nanometer 3-methyl-pentane films

An ion soft-landing approach was applied to study the motion of hydronium (D(3)O(+)) and cesium (Cs(+)) ions from 84 to 104 K in glassy 3-methyl-pentane (3MP) films vapor deposited on Pt(111). Both ions were found to have very similar mobilities in 3MP. The span of ion mobilities probed is from approximately 10(-18) to approximately 10(-13) m(2) V(-1) s(-1). Ion transport in these films was studied as a function of film thickness and electric field strength. The drift velocity was found to be linear with applied field below about 2 x 10(8) Vm and deviated from linearity above this. To a large extent, D(3)O(+) and Cs(+) motion in 3MP was well predicted by a simple continuum-based ion mobility model in films from 25 to 20,000 ML thick (including pronounced perturbations 7 ML from both the vacuum and Pt interfaces). The mobility varied with temperature more slowly than predicted by Stokes' law, which may be due to extended inhomogeneous structures in the 3MP near its glass transition at 77 K.     

乙基氰乙基纤维素/交联聚丙烯酸复合物膜的溶胀行为

研究了乙基氰乙基纤维素 [(E CE)C] 交联聚丙烯酸 [PAA]胆甾相液晶复合物膜的厚度以及膜的组成对膜在水中的溶胀行为的影响 .复合物膜越厚则达到溶胀平衡所需要的时间越长 ,但是其最大溶胀率是相同的 .复合物膜的最大溶胀率先是随着 (E CE)C浓度的增加而增加 ,当 (E CE)C的浓度大于 5 1wt%的时候 ,复合物膜的最大溶胀率几乎不再发生变化 .复合物膜的交联密度越大 ,其最大溶胀率越小 ,溶胀速率也随着膜的交联 (点 )密度的增加而减小 .研究还发现复合物膜的交联 (点 )密度越大 ,其溶胀前后最大选择性反射光波长的位移也越小 .     

Photoluminescence brightening via electrochemical trap passivation in ZnSe and Mn(2+)-doped ZnSe quantum dots

Spectroelectrochemical experiments on wide-gap semiconductor nanocrystals (ZnSe and Mn(2+)-doped ZnSe) have allowed the influence of trap electrochemistry on nanocrystal photoluminescence to be examined in the absence of semiconductor band filling. Large photoluminescence electrobrightening is observed in both materials upon application of a reducing potential and is reversed upon return to the equilibrium potential. Electrobrightening is correlated with the transfer of electrons into nanocrystal films, implicating reductive passivation of midgap surface electron traps. Analysis indicates that the electrobrightening magnitude is determined by competition between electron trapping and photoluminescence (ZnSe) or energy transfer (Mn(2+)-doped ZnSe) dynamics within the excitonic excited state, and that electron trapping is extremely fast (k(trap) ≈ 10(11) s(-1)). These results shed new light on the complex surface chemistries of semiconductor nanocrystals.     

聚(3-己基)噻吩薄膜中电荷传导的研究

对两种具有相同化学结构的聚(3-己基)噻吩膜进行了电荷传导研究以检验膜的结构对载流子迁移率的影响. 一种膜是由3-己基噻吩单体经电化学合成直接制备的膜(原位生长膜); 另一种膜是将原位生长膜溶于三氯甲烷后重新滴涂而成的(滴涂膜). 研究表明, 虽然两种膜的制备方法不一样, 但在最低(0.02%)和较高(20%～30%)掺杂率下两膜中的载流子迁移率相一致; 然而在中等掺杂率区域, 两膜中的载流子迁移率明显不同. 对于原位生长膜, 载流子迁移率在低掺杂区域几乎保持不变, 当掺杂率大于1%后开始上升; 而在滴涂膜中, 随着掺杂率的增加, 迁移率先下降然后迅速升高. 上述两种迁移率变化特征分别与以前研究中观察到的电化学合成高分子膜和化学合成高分子旋涂或滴涂膜中迁移率的变化特征相一致, 表明了迁移率随掺杂率变化特征的改变是由膜的结构变化而引起的     

A new generation of metal string complexes: structure, magnetism, spectroscopy, theoretical analysis, and single molecular conductance of an unusual mixed-valence linear [Ni5]8+ complex

Two new linear pentanickel complexes [Ni5(bna)4(Cl)2][PF6]2 (1) and [Ni5(bna)4(Cl)2][PF6]4 (2; bna=binaphthyridylamide), were synthesized and structurally characterized. A derivative of 1, [Ni5(bna)4(NCS)2][NCS]2 (3), was also isolated for the purpose of the conductance experiments carried out in comparison with [Ni5(tpda)4(NCS)2] (4; tpda=tripyridyldiamide). The metal framework of complex 2 is a standard [Ni5]10+ core, isoelectronic with that of [Ni5(tpda)4Cl2] (5). Also as in 5, complex 2 has an antiferromagnetic ground state (J=-15.86 cm(-1)) resulting from a coupling between the terminal nickel atoms, both in high-spin sate (S=1). Complex 1 displays the first characterized linear nickel framework in which the usual sequence of NiII atoms has been reduced by two electrons. Each dinickel unit attached to the naphthyridyl moieties is assumed to undergo a one-electron reduction, whereas the central nickel formally remains NiII. DFT calculations suggest that the metal framework of the mixed-valence complex 1 should be described as intermediate between a localized picture corresponding to NiII-NiI-NiII-NiI-NiII and a fully delocalized model represented as (Ni2)3+-NiII-(Ni2)3+. Assuming the latter model, the ground state of 1 results from an antiferromagnetic coupling (J=-34.03 cm(-1)) between the two (Ni2)3+ fragments, considered each as a single magnetic centre (S=3/2). An intervalence charge-transfer band is observed in the NIR spectrum of 1 at 1186 nm, suggesting, in accordance with DFT calculations, that 1 should be assigned to Robin-Day class II of mixed-valent complexes. Scanning tunnelling microscopy (STM) methodology was used to assess the conductance of single molecules of 3 and 4. Compound 3 was found approximately 40% more conductive than 4, a result that could be assigned to the electron mobility induced by mixed-valency in the naphthyridyl fragments.     

Low energy charged particles interacting with amorphous solid water layers

The interaction of charged particles with condensed water films has been studied extensively in recent years due to its importance in biological systems, ecology as well as interstellar processes. We have studied low energy electrons (3-25 eV) and positive argon ions (55 eV) charging effects on amorphous solid water (ASW) and ice films, 120-1080 ML thick, deposited on ruthenium single crystal under ultrahigh vacuum conditions. Charging the ASW films by both electrons and positive argon ions has been measured using a Kelvin probe for contact potential difference (CPD) detection and found to obey plate capacitor physics. The incoming electrons kinetic energy has defined the maximum measurable CPD values by retarding further impinging electrons. L-defects (shallow traps) are suggested to be populated by the penetrating electrons and stabilize them. Low energy electron transmission measurements (currents of 0.4-1.5 μA) have shown that the maximal and stable CPD values were obtained only after a relatively slow change has been completed within the ASW structure. Once the film has been stabilized, the spontaneous discharge was measured over a period of several hours at 103 ± 2 K. Finally, UV laser photo-emission study of the charged films has suggested that the negative charges tend to reside primarily at the ASW-vacuum interface, in good agreement with the known behavior of charged water clusters.     

Control of polymorphism in solution-processed organic thin film transistors by self-assembled monolayers

Polymorphism of organic semiconductor films is of key importance for the performance of organic thin film transistors(OTFTs).Herein, we demonstrate that the polymorphism of solution-processed organic semiconductors in thin film transistors can be controlled by finely tuning the surface nanostructures of substrates with self-assembled monolayers(SAMs). It is found that the SAMs of 12-cyclohexyldodecylphosphonic acid(CDPA) and 12-phenyldodecylphosphonic acid(Ph DPA) induce different polymorphs in the dip-coated films of 2-dodecyl[1]benzothieno[3,2-b][1]benzothiophene(BTBT-C12). The film of BTBT-C12 on CDPA exhibits field effect mobility as high as 28.1 cm~2 V~(-1) s~(-1) for holes, which is higher than that of BTBT-C12 on Ph DPA by three times. The high mobility of BTBT-C12 on CDPA is attributable to the highly oriented films of BTBT-C12 with a reduced in-plane lattice and high molecular alignment.     

A study of cadmium sulfide nanocrystalline films by grazing incidence X-ray diffraction

Thin cadmium sulfide films were prepared on a monocrystalline-crystal silicon substrate by chemical deposition from aqueous solutions. Grazing incidence X-ray diffraction revealed that the cadmium sulfide films are comprised of nanocrystal particles, with 80% of the particles having a size of 5 ± 1 nm. Some nanocrystals have a wurtzite structure, while others, a sphalerite one. The presence of cubic phase in the films is indicative of a nonequilibrium state of the nanocrystalline films. Thirty minutes after the onset of the formation of cadmium sulfide, the size and crystal structure of the constituent particles of the film become independent of the deposition time—only the film thickness increases. In addition, the initial stage of the formation of the cadmium sulfide film is accompanied by the deposition of cadmium hydroxide Cd(OH)₂.     

Metallic LiMo3Se3 nanowire film sensors for electrical detection of metal ions in water

LiMo 3Se 3 nanowire film sensors were fabricated by drop-coating a 0.05% (mass) aqueous nanowire solution onto microfabricated indium tin oxide electrode pairs. According to scanning electron microscopy (SEM) and atomic force microscopy (AFM), the films are made of a dense network of 3-7 nm thick nanowire bundles. Immersion of the films in 1.0 M aqueous solutions of group 1 or 2 element halides or of Zn(II), Mn(II), Fe(II), or Co(II) chlorides results in an increase of the electrical resistance of the films. The resistance change is always positive and reaches up to 9% of the base resistance of the films. It occurs over the course of 30-240 s, and it is reversible for monovalent ions and partially reversible for divalent ions. The signal depends on the concentration of the electrolyte and on the size and charge of the metal cation. Anions do not play a significant role, presumably, because they are repelled by the negatively charged nanowire strands. The magnitude of the electrical response and its sign suggest that it is due to analyte-induced scattering of conduction electrons in the nanowires. An ion-induced field effect can be excluded based on gated conductance measurements of the nanowire films.     

Effect of the erbium dopant architecture on the femtosecond relaxation dynamics of silicon nanocrystals

Femtosecond pump-probe absorption spectroscopy is used to investigate the role of Er(3+) dopants in the early relaxation pathways of photoexcited Si nanocrystals. The fate of photoexcited electrons in three different Si nanostructures was studied and correlated with the effect of Er-doping and the nature of the dopant architecture. In Si nanocrystals without Er(3+) dopant, a trapping component was identified to be a major electron relaxation mechanism. Addition of Er(3+) ions into the core or surface shell of the nanocrystals was found to open up additional nonradiative relaxation pathways, which is attributed to Er-induced trap states in the Si host. Analysis of the photodynamics of the Si nanocrystal samples reveals an electron trapping mechanism involving trap-to-trap hopping in the doped nanocrystals, whereby the density of deep traps seem to increase with the presence of erbium. To gain additional insights on the relative depths of the trapping sites on the investigated nanostructures, benzoquinone was used as a surface adsorbed electron acceptor to facilitate photoinduced electron transfer across the nanocrystal surface and subsequently assist in back electron transfer. The established reduction potential (-0.45 V versus SCE) of the electron acceptor helped reveal that the erbium-doped nanocrystal samples have deeper trapping sites than the undoped Si. Furthermore, the measurements indicate that internally Er-doped Si have relatively deeper trapping sites than the erbium surface-enriched nanocrystals. The electron-shuttling experiment also reveals that the back electron transfer seems not to recover completely to the ground state in the doped Si nanocrystals, which is explained by a mechanism whereby the electrons are captured by deep trapping sites induced by erbium addition in the Si lattice.     

Rational Design of Ambipolar Organic Semiconductors: Is Core Planarity Central to Ambipolarity in Thiophene–Naphthalene Semiconductors?

Herein, we report a new family of naphthaleneamidinemonoimide-fused oligothiophene semiconductors designed for facile charge transport in organic field-effect transistors (OFETs). These molecules have planar skeletons that induce high degrees of crystallinity and hence good charge-transport properties. By modulating the length of the oligothiophene fragment, the majority carrier charge transport can be switched from n-type to ambipolar behavior. The highest FET performance is achieved for solution-processed films of 10-[(2,2'-bithiophen)-5-yl]-2-octylbenzo[lmn]thieno[3',4':4,5]imidazo[2,1-b][3,8]phenanthroline-1,3,6(2H)-trione (NDI-3 Tp), with optimized film mobilities of 2×10(-2) and 0.7×10(-2) cm(2) V(-1) s(-1) for electrons and holes, respectively. Finally, these planar semiconductors are compared with their twisted-skeleton counterparts, which exhibit only n-type mobility, in order to understand the origin of the ambipolarity in this new series of molecular semiconductors.     

Direct precursor conversion reaction for densely packed Ag2S nanocrystal thin films

Successful realization of highly crystalline and densely packed Ag2S nanocrystal (NC) films has been achieved by directly converting precursor molecules, Ag(SCOPh), on preheated substrates. When an aliquot of Ag(SCOPh) solution dissolved in trioctylphosphine (TOP) is applied on preheated solid substrates at 160 degrees C, such as SiO2/Si, H-terminated Si, and quartz. Ag2S NC thin films have been formed with instant phase and color changes of the precursor solutions from pale yellow homogeneous solution to black solid films. The average diameter of individual Ag2S NCs forming thin films is ca. 25 nm, as confirmed by examining both isolated Ag2S NCs from thin films and as-made thin film samples by using transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. Powder X-ray diffraction (XRD) pattern shows that the synthesized Ag2S NCs have well-defined monoclinic acanthite phase. Direct precursor conversion process has resulted in densely packed Ag2S NCs with reduced interparticle distances owing to efficient removal of TOP during the reaction. Compared to the devices fabricated by the drop-coating process, Ag2S thin film devices fabricated by direct precursor conversion process have shown a ca. 300-fold increased conductance. Such Ag2S NC devices have also displayed reliable photoresponses upon white light illumination with high photosensitivity (S approximately equal to 1).     

Organic semiconducting materials from sulfur-hetero benzo[k]fluoranthene derivatives: synthesis, photophysical properties, and thin film transistor fabrication

A new family of air-stable sulfur-hetero oligoarenes based on the benzo[k]fluoranthene unit has been facilely developed as the active materials for thin film organic field-effect transistors. The Diels-Alder reaction between cyclopentadienone 1 and 2,2'-(ethyne-1,2-diyl)bisthiophene followed by decarbonylation afforded fluoranthene derivative 2. After bromination and subsequent substitution through Suzuki coupling reaction, the FeCl3-oxidative cyclization produced sulfur-hetero benzo[k]fluoranthene derivatives 8-12. In dilute chloroform solution, the absorption and emission behaviors of 2 and 4-7 showed characteristic features of the fluoranthene units, while their emission lambda(max) red-shifted with an increase of the effective conjugation length. The steady state absorption and emission spectra of these newly synthesized compounds were thoroughly investigated and discussed. Thin film organic field-effect transistors (OFETs) using 8-11 as active materials were fabricated in a "top contact" configuration. Substituents at the skeleton play an important role in the film morphologies, which lead to different mobilities, while the charge mobilities of 8-11 from OFETs were improved after thermal annealing of the thin films. A carrier mobility as high as 0.083 cm(2) V(-1) s(-1) and current on/off ratio of 10(6) were achieved through vacuum-deposited film followed by the thermal annealing process from 11.     

3D Printing of Drug Nanocrystals for Film Formulations

The aim of the study was to prepare indomethacin nanocrystal-loaded, 3D-printed, fast-dissolving oral polymeric film formulations. Nanocrystals were produced by the wet pearl milling technique, and 3D printing was performed by the semi-solid extrusion method. Hydroxypropyl methyl cellulose (HPMC) was the film-forming polymer, and glycerol the plasticizer. In-depth physicochemical characterization was made, including solid-state determination, particle size and size deviation analysis, film appearance evaluation, determination of weight variation, thickness, folding endurance, drug content uniformity, and disintegration time, and drug release testing. In drug nanocrystal studies, three different stabilizers were tested. Poloxamer F68 produced the smallest and most homogeneous particles, with particle size values of 230 nm and PI values below 0.20, and was selected as a stabilizer for the drug-loaded film studies. In printing studies, the polymer concentration was first optimized with drug-free formulations. The best mechanical film properties were achieved for the films with HPMC concentrations of 2.85% (w/w) and 3.5% (w/w), and these two HPMC levels were selected for further drug-loaded film studies. Besides, in the drug-loaded film printing studies, three different drug levels were tested. With the optimum concentration, films were flexible and homogeneous, disintegrated in 1 to 2.5 min, and released the drug in 2–3 min. Drug nanocrystals remained in the nano size range in the polymer films, particle sizes being in all film formulations from 300 to 500 nm. When the 3D-printed polymer films were compared to traditional film-casted polymer films, the physicochemical behavior and pharmaceutical performance of the films were very similar. As a conclusion, 3D printing of drug nanocrystals in oral polymeric film formulations is a very promising option for the production of immediate-release improved- solubility formulations.     

Synthesis and characterization of nanostructured bismuth selenide thin films

Nanostructured bismuth selenide thin films have been successfully fabricated on a silicon substrate at low temperature by rational design of the precursor solution. Bi(2)Se(3) thin films were constructed of coalesced lamella in the thickness of 50-80 nm. The nucleation and growth process of Bi(2)Se(3) thin films, as well as the influence of solution chemistry on the film structure were investigated in detail. As one of the most promising thermoelectric materials, the thermoelectric properties of the prepared Bi(2)Se(3) thin films were also investigated. The power factor increased with increasing carrier mobility, coming from the enlarged crystallites and enhanced coalesced structure, and reached 1 μW cm(-1) K(-1).