Nanoscale measurements of conducting domains and current-voltage characteristics of chemically deposited polyaniline films

Spatial variations in electric conductivity and evolutions of band structures of polyaniline (PANI) films have been studied by use of a so-called current-sensing atomic force microscope (CS-AFM) or atomic force microscope current image tunneling spectroscopy (AFM-CITS). PANI films were deposited chemically onto indium-tin oxide- (ITO-) glass substrates, and their thickness and doping levels were controlled by polymerization and acid-doping conditions. The conducting uniformity of the PANI films depends on their doping level and thickness. Conducting domains were observed in fully doped PANI film, even when the bias voltage was reduced to as small as 30 mV. High current flowing regions gradually disappeared when conducting PANI films were partially dedoped. The point-contact current-voltage (I-V) characteristics of conducting tip-polymer/ITO systems were investigated on PANI films with different thickness and degree of doping. Various types of I-V curves representing metallic, semiconducting, and insulating states were obtained depending on the aggregation of polymer chains and doping level of the polymer film. The band gap energies (estimated from the I-V or dI/dV-V curves) of emeraldine base (EB) (undoped polyaniline) films are all higher than 3.8 eV, and a wide distribution of the band gap energies (0-1.1 eV and 0.75-1.8 eV for fully and partially doped PANI thin films, respectively) was found in a single polymer film.     

Effects of polymerization media on the nanoscale conductivity and current-voltage characteristics of chemically synthesized polyaniline films

A current sensing atomic force microscope (CS-AFM) was used to probe the conducting homogeneity and band structures of fully doped polyaniline (PANI) films prepared from in situ chemical polymerization/deposition of aniline on indium tin oxide in various inorganic acids. The charge transport properties of PANI films depend on the film thickness as well as polymerization medium. Fluctuations in conductivity are observed on all acid-doped PANI films and the conducting homogeneity was dependent on the film thickness: the conductivity of thick film is more uniform. The current-voltage (I-V) characteristics of all thick (>200 nm) films displayed a metal-like behavior and conductivity as high as 40 S/cm was detected in high conducting regions of film thicker than 400 nm. Whereas thin (<120 nm) films revealed insulating, semiconducting, and semimetal conducting, wide distribution in conductivity and interband distances (estimated from the I-V ordI/dV-V curves) was found. The interband distances is 0-1.35, 0-1.0, and 0-0.78 eV for thin PANI film prepared from HCl(aq), HClO(4)(aq), and H2SO4(aq), respectively. PANI film (260 nm) prepared from H2SO4(aq) revealed fiberlike morphology, and compared to PANI films prepared from HCl(aq) and HClO4(aq) with similar thickness, it has higher average nanoscale conductivity but lower bulk conductivity. This result could be direct evidence which supports that the bulk conductivity of PANI depended on the carriers hopping between the conducting domains.     

脉冲激光沉积制备SrSn1-xCoxO3外延薄膜的微结构表征及能带调控

钙钛矿结构SrSnO₃因其独特的介电和半导体性质而备受关注,通过掺杂可显著调控其电学、磁学性能,拓宽其应用范围。本研究在单晶SrTiO₃(001)衬底上通过脉冲激光方法外延生长了SrSn_1-xCo_xO₃ (x = 0, 0.16, 0.33, 0.5) (SSCO)薄膜,探究了Co含量对薄膜结晶性、微观结构、光学性能以及介电性能的影响。结果表明, SrSn_1-xCo_xO₃薄膜可在SrTiO₃(001)衬底上外延生长, Co掺杂不会导致薄膜结晶质量的劣化。薄膜表面形貌平整、致密,膜厚200 nm,表面粗糙度为0.44 nm。随薄膜中Co掺杂量增加,薄膜透过率从90%降至25%,光学带隙从4.24 eV降至2.44 eV。介电性能测试表明,掺杂薄膜在10⁶Hz时介电常数为70.1,比无掺杂SrSnO₃薄膜提高57%。室温时SSCO薄膜表面电阻率为172 MΩ,在1000℃范围内薄膜结构稳定。     

Electrical bistability in electrostatic assemblies of CdSe nanoparticles

We report electrical bistability in electrostatic assembly of CdSe nanoparticles. We obtained thin films of the nanoparticles via layer-by-layer electrostatic assembly technique, which provided a nanoscale control to tune the thickness. Devices based on such thin films exhibit electrical bistability along with memory phenomenon. The bistability is due to charge confinement in the nanoparticles. Conduction mechanism changes from an injection-dominated to a bulk one during switching from a low- to a high-conducting state. Additionally, results from impedance spectroscopy show that the dielectric constant of the material increases during the transition. Both random-access and read-only memory applications are observed in these systems.     

Interfacial phenomena in polymer films and generation of maxwell displacement current

Interfacial electrostatic phenomena in ultrathin polyimide films have been examined, and the space charge distribution and electronic density of states have been determined. The presence of excess negative charges at the film-metal interface of nanometer thickness has been revealed and the alignment of the surface Fermi level of polymer films and Fermi level of metals have been elucidated. Taking into account the interfacial space charge, a step structure observed in the I-V characteristic of metal-polyimide-rhodamine-polyimide-metal junction, very similar to Coulomb staircase, is well explained. Furthermore, the electrical breakdown mechanism of a nanometer-thick polyimide film is found quite different from that of micrometer-thick films, owing to the presence of this interfacial nanometric space charge. Finally, for a profound understanding of the behaviour of surface monolayer, the Maxwell displacement current measurement coupled with optical second harmonic generation measurement has been employed.     

Differential conductance switching of planar tunnel junctions mediated by oxidation/reduction of functionally protected ferrocene

Planar tunnel junctions were fabricated by self-assembling 1,1'- ferrocenedicarboxylic acid (FDCA) onto native oxides of thermally deposited aluminum films and subsequently depositing a second aluminum film. Junctions were characterized using Reflection-Absorption Fourier Transform Infrared Spectroscopy (RAIRS) and current-voltage (I-V) spectroscopy. Before deposition of the second aluminum film, RAIRS of FDCA and ferrocenecarboxylic acid (FCA) films revealed COO(-), C=O, and Fc ring stretching modes, indicating that both types of molecules can interact strongly with the oxide and remain intact. After deposition, systems exhibited prominent COO(-) modes and weakened C=O modes, indicating further reaction with aluminum/aluminum oxide. Fc ring modes persisted in FDCA systems but disappeared in FCA systems, suggesting that the second COOH group in the FDCA molecule can act as a protecting group for the ferrocene moiety. Cyclic I-V measurements of FDCA tunnel junction systems revealed very strong ( approximately 10-fold) hysteretic differential conductance switching that was both reversible and stable. Control measurements using as prepared junctions, as well as junctions containing 1,6-hexanedioic acid, 1,9-nonanedioic acid, 1,4-dibenzoic acid, or FCA revealed only very weak ( approximately 10%) differential conductance changes. We attribute FDCA junction switching to barrier profile modifications induced by oxidation/reduction of the functionally protected ferrocene moieties.     

Characteristics of SnO2 thin films prepared by SILAR

SnO₂ thin films were deposited on glass substrates by using Successive Ionic Layer Adsorption and Reaction (SILAR) method at room temperature. The film thickness effect on characteristic parameters such as structural, morphological, optical and electrical properties of the films was studied. Also, the films were annealed in oxygen atmosphere (400 °C, 30 min) and characteristic parameters of the films were investigated. The X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) studies showed that all the films exhibited polycrystalline nature with tetragonal structure and were covered well on glass substrates. After the investigation of the crystalline and surface properties of the films, it was found that they were improving with increasing film thickness. Optical band gap decreased from 3.90 eV to 3.54 eV and electrical conductivity changed between 0.015–0.815 (Ω-cm)⁻¹ as the film thickness increased from 215 to 490 nm. The refractive index (n), optical static and high frequency dielectric constants (ɛ_o, ɛ_∞) values were calculated by using the optical band gap values as a function of the film thickness.     

Spectroscopic and electrical properties of SiO2 films prepared by simple and cost effective sol-gel process

Amorphous SiO2 thin films were prepared on glass and silicon substrates by cost effective sol-gel method. Tetra ethyl ortho silicate (TEOS) was used as the precursor material, ethanol as solvent and concentrated HCl as a catalyst. The films were characterized at different annealing temperatures. The optical transmittance was slightly increased with increase of annealing temperature. The refractive index was found to be 1.484 at 550 nm. The formation of SiO2 film was analyzed from FT-IR spectra. The MOS capacitors were designed using silicon (100) substrates. The current-voltage (I-V), capacitance-voltage (C-V) and dissipation-voltage (D-V) measurements were taken for all the annealed films deposited on Si (100). The variation of current density, resistivity and dielectric constant of SiO2 films with different annealing temperatures was investigated and discussed for its usage in applications like MOS capacitor. The results revealed the decrease of dielectric constant and increase of resistivity of SiO2 films with increasing annealing temperature.     

PMMA/超支化聚(酯-酰胺)共混物薄膜中柱状相结构的形成

利用相差显微镜、三维形貌测量仪对聚甲基丙烯酸甲酯 超支化聚 (酯 酰胺 ) (PMMA HBP)共混物薄膜在玻璃基板作用下的相分离行为进行了研究 .结果表明 ,不同组分比的共混物薄膜会呈现不同的相形态和相分离过程 .当薄膜厚度在 5 0 0nm左右 ,HBP为低组分时 ,发现了一种特殊的分散相为圆柱状的相形态 ,并对该相形态出现的条件进行了研究 .认为基板与组分之间的相互作用和薄膜厚度决定了圆柱状结构形成 .     

硫氰酸银钾复合薄膜的制备及其电存储特性

采用真空热蒸镀法在银电极上蒸镀硫氰酸钾薄膜,并通过界面反应在银电极表面上形成AgK2(SCN)3复合薄膜.采用可见光谱、X射线光电子能谱(XPS)、激光拉曼光谱和X射线多晶衍射谱(XRD)对薄膜进行表征.研究发现,Al/AgK2(SCN)3/Ag器件具有稳定的可逆电双稳特性,高、低电阻状态的电阻比高达106,并能实现连续"写-读-擦-读"操作.器件的可逆擦写特性归因于外电场作用下AgK2(SCN)3复合介质层内导电通道的形成-断裂;电流-电压曲线拟合显示,低电阻状态符合欧姆传输,而高电阻状态表现出空间电荷限制电流传输模式.在导电通道断裂的过程中,电离作用和焦耳热效应会共同起作用.     

Ga对在AlN衬底上直接生长石墨烯的远程催化

通过镓(Ga)远程催化,采用化学气相沉积(CVD)方法在氮化铝(AlN)衬底上直接生长石墨烯薄膜.研究了生长温度、催化剂距离对石墨烯生长及其光学性质和电学性质的影响规律.结果表明,在生长温度1070℃下可以制备厚度约为5层的石墨烯薄膜, Ga周围1.4 cm范围内可以得到厚度均匀的石墨烯薄膜.通过透光率和方阻表征了石墨烯的光学和电学性质,结果表明, 400～800 nm波长范围内石墨烯薄膜透光率可达90%以上,方阻约为230Ω/.第一性原理计算结果表明,石墨烯仍保持金属性, AlN衬底对石墨烯有吸附掺杂作用,可有效降低石墨烯的方阻,改善石墨烯和衬底的电学接触.     

Electrochemical characterization of nanoporous films fabricated from a polystyrene-poly(methylmethacrylate) diblock copolymer: monitoring the removal of the PMMA domains and exploring the functional groups on the nanopore surface

Cyclic voltammetry (CV) was used to assess fabrication of a nanoporous film from a polystyrene-poly(methyl methacrylate) diblock copolymer (PS-b-PMMA) and also to explore the surface functional groups on the resulting nanopores. Polymer films containing vertically aligned cylindrical nanoscale pores (ca. 10 nm in pore radius, 20-30 nm in film thickness) were prepared on gold substrates by removing the cylindrical PMMA domains from PS-b-PMMA films via UV irradiation and subsequent acetic acid treatment. CV measurements provided a simple means for monitoring the extent of the removal of the PMMA domains and for assessing the formation of a recessed nanodisk-array electrode (RNE) structure. The resulting RNEs exhibited a decrease in redox current of anionic Fe(CN)6(3-) with increasing solution pH from 4.6 to 6.3 and a negligible change in CV of uncharged 1,1'-ferrocenedimethanol. The decrease in redox current of Fe(CN)6(3-) at the higher pH was due to electrostatic repulsion between Fe(CN)6(3-) and the electrical double layer formed in the neighborhood of the negatively charged nanopore surface. Indeed, the reduction of effective pore radius measured from CVs of Fe(CN)6(3-) was correlated to the change in the thickness of the electrical double layer. The pH range that showed the decrease in redox current of Fe(CN)6(3-) was consistent with the presence of -COOH groups on the nanopore surface, although they were not detected using Fourier transform infrared spectra of etched PS-b-PMMA films.     

Nanopatterning and fabrication of memory devices from layer-by-layer poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) ultrathin films

A write-read-erasable memory device was fabricated on layer-by-layer (LbL) ultrathin films prepared from poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT-PSS) and poly(diallyldimethylammonium chloride) (PDDA). By use of current-sensing atomic force microscopy (CS-AFM), nanopatterns were formed by applying a bias voltage between a conductive tip (Pt-coated Si3N4 cantilever) in contact with the polymer film and gold substrate. The dependence of the nanopatterns on film thickness, applied bias, and writing speed was studied. Moreover, the height of the patterns was 3-5 times higher than the original thickness of the films, opening the possibility for three-dimensional nanopatterning. The ability of the patterns to be erased after nanowriting was also investigated. By comparing the I-V characteristics under ambient conditions and under N2 environment, a joule-heating activated, water meniscus-assisted anion doping mechanism for the nanopatterning process was determined. Write-read-erase memory device capability was demonstrated on the nanopatterns.     

Electric-field-induced rupture of polymer-stabilized oil films

Water-in-oil emulsions stabilized by polymeric surfactants are robust, but the reasons for their stability are poorly understood. We studied oil films stabilized by a comb–graft copolymer having a poly(siloxane) backbone and poly(ethylene oxide)/poly (propylene oxide) and C₁₆ grafts (Abil EM-90) with a total number-average molecular weight of 62,000. Electric fields imposed in the aqueous phases on either side of the oil films were used to induce rapid rupture, and the response of the film was monitored using optical interference and electrical conductance measurements. Film thickness values ranged between 30 and 50 nm and rupture at field strength values between 2 × 10⁷ and 5 × 10⁷ V/m. Unexpectedly, in some cases, stable pores were formed and the films became electrically conductive. Often the pores persisted for more than 20 min after the voltage had been removed. Since the current was independent of film area, very few pores are involved in conduction. This behavior is similar to that found in lipid films; however, the persistence time is greater for polymer-stabilized films. Because the films are thick, it is possible that pores are formed by multimolecular self-assembly as with pore-forming proteins. Polymer purification also influenced film stability. Received: 4 February 1999 Accepted: 21 May 1999     

Donor–Acceptor Oligoimides for Application in High‐Performance Electrical Memory Devices

Two new oligoimides, OI(APAP-6FDA) and OI(APAN-6FDA) , which consisted of electron‐donating N‐(4‐aminophenyl)‐N‐phenyl‐1‐aminopyrene ( APAP ) or N‐(4‐aminophenyl)‐N‐phenyl‐1‐aminonaphthalene ( APAN ) moieties and electron‐accepting 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride ( 6FDA ) moieties, were designed and synthesized for application in electrical memory devices. Such devices, with the indium tin oxide (ITO)/oligoimide/Al configuration, showed memory characteristics, from high‐conductance Ohmic current flow to negative differential resistance (NDR), with corresponding film thicknesses of 38 and 48 nm, respectively. The 48 nm oligoimide film device exhibited NDR electrical behavior, which resulted from the diffusion of Al atoms into the oligoimide layer. On further increasing the film thickness to 85 nm, the OI(APAP-6FDA) film device showed a reproducible nonvolatile “write once read many” (WORM) property with a high ON/OFF current ratio (more than ×10⁴). On the other hand, the device that was based on the 85 nm OI(APAN-6FDA) film exhibited a volatile static random access memory (SRAM) property. The longer conjugation length of the pyrene unit compared to that of a naphthalene unit was considered to be responsible for the different memory characteristics between these two oligoimides. These experimental results suggested that tunable switching behavior could be achieved through an appropriate design of the donor–acceptor oligoimide structure and controllable thickness of the active memory layer.     

Influence of Poly (methyl metacrylate) Addition on Resistive Switching Performance of P3HT/P(VDF-TrFE) Blend Films

Organic semiconducting/ferroelectric blend films attracted much attention due to their elec-trical bistability and rectification properties and thereof the potential in resistive memory devices. Blend films were usually deposited from solution, during which phase separation oc-curred, resulting in discrete semiconducting phase whose electrical property was modulated by surrounding ferroelectric phase. However, phase separation resulted in rough surface and thus large leakage current. To further improve electrical properties of such blend films, poly(methyl metacrylate) (PMMA) was introduced as additive into P3HT/P(VDF-TrFE) semiconducting/ferroelectric blend films in this work. It indicated that small amount of PMMA addition could effectively enhance the electrical stability to both large electrical stress and electrical fatigue and further improve retention performance. Overmuch PMMA addition tended to result in the loss of resistive switching property. A model on the con-figuration of three components was also put forward to well understand our experimental observations.     

Optical Switching in VO2 Thin Films

Vanadium dioxide thin films have been deposited from vanadium alkoxides VO(OR)3. An amorphous film is formed that transforms into crystalline VO2 upon heating at 500°C under a reducing atmosphere. Optically transparent VO2 thin films are then obtained that exhibit both electrical and optical switching around 70°C. The switching temperature together with the shape of the hysteresis loop can be modified by doping VO2 films with foreign cations. Doped MxVO2 (M = W6+, Nb5+, Ti4+, Cr3+ or Al3+) thin films have been prepared under the same conditions by mixing the vanadium alkoxide and a metal salt in an alcoholic solution. The switching temperature decreases when the film is doped with high-valent cations (W6+) and increases with low-valent cations (Al3+, Cr3+). The transition temperature first decreases and then increases when TiIV is added to the VO2 film while the width of the hysteresis loop is significantly reduced.     

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     

钛金属有机物热解制备TiO2-SiO2复合膜及其光催化活性研究

混有一定量SiO_2溶胶的钛金属有机化合物膜液通过旋液成膜法制备前驱物膜 ，经热解得到TiO_2-SiO_2复合膜。于610 ℃焙烧15min所得复合膜(Ti:Si=9:1)经 SEM，XRD，UV-vis和XPS研究表明，膜面由30 nm * 200 nm大小的晶体粒子组成， 结构致密，膜厚约200 nm，其可见光透过率为玻璃基质的80%，膜表面Ti~(3+)OH~- 的比值为1.06。对不同SiO_2含量的膜液凝胶进行DSC分析显示，少量的SiO_2就能 显著提高TiO_2锐钛矿型晶相的形成温度。膜的光催化活性研究表明一定量的Fe~ (3+)有利于提高膜的光催化活性，但是如果以氯化物的形式加入则对光催化反应不 利，铬的氯化物同样如此。另外，在钛金属有机物热解制备TiO_2-SiO_2复合膜中 ，溶胶SiO_2不利于光催化反应，但是它可以改善膜的耐磨性。     

Thickness dependent physical properties of close space evaporated In2S3 films

In recent years, In₂S₃ is considered as a promising buffer layer in the fabrication of heterojunction solar cells. Film thickness is one of the important parameters that alters the physical characteristics of the grown layers significantly. The effect of film thickness on the structural, morphological, optical and electrical properties of close space evaporated In₂S₃ layers has been studied. In₂S₃ thin films with different thicknesses in the range, 100–700 nm were deposited on Corning glass substrates at a constant substrate temperature of 300 °C. The films were polycrystalline exhibiting strong crystallographic orientation along the (103) plane. The deposited films showed mixed phases of both cubic and tetragonal structures up to a thickness of 300 nm. On further increasing the film thickness, the layers showed only tetragonal phase. With increase of film thickness, both the crystallite size and surface roughness in the films were found to be increased. The optical constants such as refractive index and extinction coefficient of the as-grown layers have been calculated from the optical transmittance data in the wavelength range, 300–2500 nm. The optical transmittance of the films was decreased from 82% to 64% and the band gap varied in the range, 2.65–2.31 eV with increase of film thickness. The electrical resistivity as well as the activation energy was evaluated and found to decrease with film thickness. The detailed study of these results was presented and discussed.