Characterization of Geoinspired and Synthetic Chrysotile Nanotubes by Atomic Force Microscopy and Transmission Electron Microscopy

Geoinspired synthetic chrysotile nanotubes both stoichiometric and 0.67 wt % Fe doped were characterized by transmission electron microscopy and electron diffraction. Bending tests of the synthetic chrysotile nanotubes were performed using the atomic force microscope. The nanotubes were found to exhibit elastic behaviour at small deformations (below ca. 20 nm). Young's modulus values of (159 ± 125) GPa and (279 ± 260) GPa were obtained from the force‐deflection curves using the bending equation for a clamped beam under a concentrated load, for the stoichiometric and the Fe doped chrysotile nanotubes, respectively. The structural modifications induced by Fe doping altered the mechanical properties, with an apparent dependence of the latter on the number of constituting walls of the nanotubes.     

STM光纤探针的研制

以普通的石英光纤为材料运用腐蚀－熔拉－腐蚀复合的方法制备出光纤探针，而后在针尖表面镀上一层数十nm厚的金属膜。达到导电性，使其能传导隧道电镜，并在STM上取得了理想的测试结果。     

Force Spectroscopic Investigations During the Local Oxidation of n‐Octadecyltrichlorosilane Monolayers

Scanning force spectroscopy (SFS) is a powerful tool for investigating surface properties with high precision. Unlike most common spectroscopic techniques, information about local properties can also be obtained from surface areas with nanometer dimensions. This makes SFS a useful investigative tool for small lithographic structures. We apply the continuous recording of force curves to extract valuable information about the local oxidation of a monolayer of n‐octadecyltrichlorosilane molecules self‐assembled on silicon. The oxidation is carried out while simultaneously recording the force curves during the application of a bias voltage to the tip. The dynamics of the induced surface modifications and changes in the surface properties are followed by analyzing specific spots in the force curves.     

A Kelvin Probe Force Microscopy Study of the Photogeneration of Surface Charges in All‐Thiophene Photovoltaic Blends

Light‐induced generation of charges into an electron acceptor–donor phase‐segregated blend is studied. The blend is made of highly ordered nanoscopic crystals of 3″‐methyl‐4″‐hexyl‐2,2′:5′,2″:5″,2?:5?,2″″‐quinquethiophene‐1″,1″‐dioxide embedded into a regioregular poly(3‐hexylthiophene) matrix, acting as acceptor and donor materials, respectively. Kelvin probe force microscopy investigations reveal a tendency for the acceptor nanocrystals to capture the generated electrons whereas the donor matrix becomes more positively charged. The presence of particular positively charged defects, i.e., nanocrystals, is also observed within the film. The charging and discharging of both materials is studied in real time, as well as the effect of different acceptor–donor ratios. Upon prolonged thermal annealing at high temperatures the chemical structure of the blend is altered, leading to the disappearance of charge separation upon light irradiation. The obtained results allow a better understanding of the correlation between the nanoscopic structure of the photoactive material and solar‐cell performance.     

Macro-scale atomic force microscope: An experimental platform for teaching precision mechatronics

A macro-scale atomic force microscope (macro-AFM) has been designed and used for teaching precision mechatronics. The macro-AFM uses a novel electromagnetic self-sensing self-actuating probe. It operates in frequency-modulation AFM (FM-AFM) mode with intermittent contact. The AFM has an imaging volume of 250 mm × 40 mm × 1 mm with a resolution of about 1 μm at 1 Hz measurement bandwidth. The macro-AFM is simple and relatively inexpensive to build. It is scaled to make motion visible to the eye and is robust for a lab environment, all of which make it an affordable and effective educational tool. The macro-AFM is used in Mechatronics (2.737), a graduate level course at MIT, where in a series of 11 laboratory exercises, the students assembled and programmed the AFM system. This article provides the design and theory used for making and controlling the macro-AFM, as well as experimental results.     

Phase Segregation in Thin Films of Conjugated Polyrotaxane– Poly(ethylene oxide) Blends: A Scanning Force Microscopy Study

Scanning force microscopy (SFM) is used to study the surface morphology of spin‐coated thin films of the ion‐transport polymer poly(ethylene oxide) (PEO) blended with either cyclodextrin (CD)‐threaded conjugated polyrotaxanes based on poly(4,4′‐diphenylene‐vinylene) (PDV), β‐CD–PDV, or their uninsulated PDV analogues. Both the polyrotaxanes and their blends with PEO are of interest as active materials in light‐emitting devices. The SFM analysis of the blended films supported on mica and on indium tin oxide (ITO) reveals in both cases a morphology that reflects the substrate topography on the (sub‐)micrometer scale and is characterized by an absence of the surface structure that is usually associated with phase segregation. This observation confirms a good miscibility of the two hydrophilic components, when deposited by using spin‐coating, as suggested by the luminescence data on devices and thin films. Clear evidence of phase segregation is instead found when blending PEO with a new organic‐soluble conjugated polymer such as a silylated poly(fluorene)‐alt‐poly(para‐phenylene) based polyrotaxane (THS–β‐CD–PF–PPP). The results obtained are relevant to the understanding of the factors influencing the interfacial and the intermolecular interactions with a view to optimizing the performance of light‐emitting diodes, and light‐emitting electrochemical cells based on supramolecularly engineered organic polymers.     

Conductivity of SU‐8 Thin Films through Atomic Force Microscopy Nano‐Patterning

Processing flexibility and good mechanical properties are the two major reasons for SU‐8 extensive applicability in the micro‐fabrication of devices. In order to expand its usability down to the nanoscale, conductivity of ultra‐thin SU‐8 layers as well as its patterning by AFM are explored. By performing local electrical measurements outstanding insulating properties and a dielectric strength 100 times larger than that of SiO₂ are shown. It is also demonstrated that the resist can be nano‐patterned using AFM, obtaining minimum dimensions below 40nm and that it can be combined with parallel lithographic methods like UV‐lithography. The concurrence of excellent insulating properties and nanometer‐scale patternability enables a valuable new approach for the fabrication of nanodevices. As a proof of principle, nano‐electrode arrays for electrochemical measurements which show radial diffusion and no overlap between different diffusion layers are fabricated. This indicates the potential of the developed technique for the nanofabrication of devices.     

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.     

超光滑光学基底表面原子力显微镜测试方法

原子力显微镜(AFM)是评价亚纳米级表面粗糙度σRMS最主要的测试仪器,但其测试结果会因采样条件(采样间距、采样点数)及测量点位置变化而改变。以AFM测试超光滑光学基底随机表面为例,应用累积功率谱理论建立了确定合理采样条件的方法,避免了采样条件选取不当带来的数据丢失或冗余;通过全局优化选取测量点和局部优化选取测量点相结合,降低了样品表面区域性差异给测试结果带来的不确定性,并大大减少了获得可靠测试结果所需的测试量。上述工作为超光滑光学基底AFM测试提供了有效方案。     

基于原子力显微镜的微桥机械特性测量

研究了基于原子力显微镜(AFM)的微桥机械特性的测量方法。通过微机电系统(MEMS)技术制备了可用静电力驱动进行机械振动的金属微桥。利用一套改进的商用AFM实验装置对测量方法进行了优化,并对微桥的共振频率进行了测量,所得实验结果与理论估算和仿真计算的结果基本一致。基于AFM的微桥机械特性的测量具有精度高和容易实现的特点,可作为测量平台扩展用于薄膜材料或微量液体的内耗、粘弹等性质的表征。     

Wear‐Resistant Nanoscale Silicon Carbide Tips for Scanning Probe Applications

The search for hard materials to extend the working life of sharp tools is an age‐old problem. In recent history, sharp tools must also often withstand high temperatures and harsh chemical environments. Nanotechnology extends this quest to tools such as scanning probe tips that must be sharp on the nanoscale, but still very physically robust. Unfortunately, this combination is inherently contradictory, as mechanically strong, chemically inert materials tend to be difficult to fabricate with nanoscale fidelity. Here a novel process is described, whereby the surfaces of pre‐existing, nanoscale Si tips are exposed to carbon ions and then annealed, to form a strong silicon carbide (SiC) layer. The nanoscale sharpness is largely preserved and the tips exhibit a wear resistance that is orders of magnitude greater than that of conventional silicon tips and at least 100‐fold higher than that of monolithic, SiO‐doped diamond‐like‐carbon (DLC) tips. The wear is well‐described by an atom‐by‐atom wear model, from which kinetic parameters are extracted that enable the prediction of the long‐time scale reliability of the tips.     

Quantitative Measurement of the Local Surface Potential of π‐Conjugated Nanostructures: A Kelvin Probe Force Microscopy Study

We describe a systematic study on the influence of different experimental conditions on the Kelvin probe force microscopy (KPFM) quantitative determination of the local surface potential (SP) of organic semiconducting nanostructures of perylene‐bis‐dicarboximide (PDI) self‐assembled at surfaces. We focus on the effect of the amplitude, frequency, and phase of the oscillating voltage on the absolute surface potential value of a given PDI nanostructure at a surface. Moreover, we investigate the role played by the KPFM measuring mode employed and the tip–sample distance in the surface potential mapping by lift‐mode KPFM. We define the ideal general conditions to obtain a reproducible quantitative estimation of the SP and we find that by decreasing the tip–sample distance, the area of substrate contributing to the recorded SP in a given location of the surface becomes smaller. This leads to an improvement of the lateral resolution, although a more predominant effect of polarization is observed. Thus, quantitative SP measurements of these nanostructures become less reliable and the SP signal is more unstable. We have also devised a semi‐quantitative theoretical model to simulate the KPFM image by taking into account the interplay of the different work functions of tip and nanostructure as well as the nanostructure polarizability. The good agreement between the model and experimental results demonstrates that it is possible to simulate both the change in local SP at increasing tip–sample distances and the 2D potential images obtained on PDI/highly oriented pyrolytic graphite samples. These results are important as they make it possible to gain a quantitative determination of the local surface potential of π‐conjugated nanostructures; thus, they pave the way towards the optimization of the electronic properties of electroactive nanometer‐scale architectures for organic (nano)electronic applications.     

Capillary Force Lithography: Fabrication of Functional Polymer Templates as Versatile Tools for Nanolithography

The implementation of high‐resolution polymer templates fabricated by capillary force lithography (CFL) is explored both in nanoimprint lithography (NIL) and in the wet‐etching of metals. Several different thermoplastic and UV‐curable polymers and types of substrates are incorporated into the general CFL procedure to meet the diverging requirements of these two applications. The mechanical stability of UV‐curable templates for imprinting in polymers, as examined by atomic force microscopy (AFM), and their anti‐adhesive properties are excellent for application in NIL. The conditions for curing the UV‐curable polymer are optimized in order to obtain high‐stability polymer templates. Gold patterns on silicon with a lateral resolution of 150 nm are fabricated by subsequent lift‐off in acetone. Similar patterns with a lateral resolution of 100 nm are fabricated using templates of thermoplastic polymers on gold layers on silicon as an etch mask. The transfer of stamp residues during CFL with these polymer templates is proven by X‐ray photoelectron spectroscopy (XPS) and AFM friction analysis. For poly(methylmethacrylate) (PMMA), the presence of large amounts of silicon‐containing residues is found to compromise the processability of the resulting template in subsequent O₂ reactive‐ion etching (RIE) treatment. The extent of silicon contamination is up to six times less for polystyrene (PS). At this level, the etch performance of the PS etch mask is not affected, as was the case for PMMA. Accurate downscaling of the lateral dimensions of the resulting metal patterns by several factors with respect to the dimensions of the PS etch mask is achieved by over‐etching of the gold. Overall, the results in this paper demonstrate the potential of CFL templates as tools for high‐resolution soft lithography.     

基于超平表面的原子力显微镜探针磨损研究

原子力显微镜的图像质量受到探针磨损情况的直接影响,本文主要对基于超平表面的原子力显微镜(AFM)探针磨损问题进行了试验研究,针对探针磨损效率以材料表面的粗糙度(Rq)作为评估指标,测试样品选用了Rq小于0.5nm的超平表面。探针在10%的反馈回路设定值比例和悬臂目标振幅比例的测试条件下,采用1Hz的扫描速度及1.7的I-gain值可使磨损最小,进而有效提升探针使用寿命。     

Exploiting Chemical Switching in a Diels–Alder Polymer for Nanoscale Probe Lithography and Data Storage

Reversibly crosslinked polymer films have properties that are beneficial to scanned‐probe data storage and lithographic applications that use thermomechanical nanoindentation as a write or expose mechanism. The novel polymer under study contains linkages based on thermally reversible Diels–Alder crosslinking. Thermomechanical properties on the nanometer scale are analyzed by indentation experiments on polymer thin films using heated tips. The underlying indentation mechanism is studied at varying tip temperatures and indentation times, revealing Arrhenius kinetics. This is in contrast to the Williams–Landau–Ferry kinetics usually observed for polymer systems. The discrepancy is explained by the reversible crosslinking incorporated into the structure of the polymer that allows switching between two different states: a rigid, highly crosslinked, low‐temperature state, and a deformable, fragmented, high‐temperature state. An individual indentation volume of less than 10^–20 L (10 000 molecule pairs) is estimated. These kinetics experiments demonstrate that a chemical reaction of only a few thousand molecules can be transduced into a mechanically measurable action. The ability to cycle between two sets of properties in these materials opens up new perspectives in lithography and data storage. Examples of data storage with densities up to 1 Tb in.^–2 and maskless lithography with resolution below 20 nm are demonstrated at writing times of 10 μs per bit/pixel.     

光刻探头的共振压力显微系统的研究

以磁共振压力显微和半导体核子自旋态研究为目的,采用电子束和等离子体刻印方法制备硅振荡器,脉冲调制序列控制磁共振条件频率和旋转框架翻转,依据样品的自旋-晶格弛豫时间和自旋-自旋弛豫时间,获得了共振显微压力,测试了压力显微的灵敏度。结果表明,光刻的探头具有高Q值和软悬臂低倔强系数K,磁共振压力通过扫描片段和激光光纤干涉得到了极其微小的压力。光刻探头的共振压力显微具有高空间分辨率,兼具核磁共振成像(MRI)和原子压力显微技术(AFM)优点,是一种重要的核自旋探测技术和三维原子分辨率成像的有力方法。     

Inside Front Cover: Oxidation Conditions for Octadecyl Trichlorosilane Monolayers on Silicon: A Detailed Atomic Force Microscopy Study of the Effects of Pulse Height and Duration on the Oxidation of the Monolayer and the Underlying Si Substrate (Adv. Funct. Mater. 6/2005)

Schubert and co‐workers have performed a detailed atomic force microscopy study to establish and characterize the oxidation conditions of self‐assembled monolayers of octadecyl trichlorosilane on silicon substrates. The cover image illustrates different examples of surfaces that were structured with different patterning conditions, as reported on p. 938. The graph in the background depicts three observed oxidation regimes depending on applied voltage and oxidation time. In current scanning‐probe nanolithography research, substrates consisting of octadecyl trichlorosilane monolayers on silicon are often used. On one hand, the presence of an organic monolayer can be used as a passive resist, influencing the formation of silicon dioxide on the substrate, whereas in other cases the monolayer itself is patterned, creating local chemical functionality. In this study we investigate the time scales involved in either process. By looking at friction and height images of lines oxidized at different bias voltages and different pulse durations, we have determined the parameter space in which the formation of silicon dioxide is dominant as well as the region in which the oxidation of the monolayer itself is dominant.     

摩擦强度对薄膜表面形态的作用:原子力显微镜下的观察

展示了摩擦强度对聚酰亚胺薄膜表面形态的影响，原子力显微图像显示，机械摩擦会使聚酰亚胺薄膜表面上形成微沟槽，这些沟槽的表面具有丰富的表面精细构造。原子显微图像还揭示了机械摩擦可以改变被磨擦聚酰亚胺膜的表面形态。     

Nanostructure and Optoelectronic Characterization of Small Molecule Bulk Heterojunction Solar Cells by Photoconductive Atomic Force Microscopy

Photoconductive atomic force microscopy is employed to study the nano­scale morphology and optoelectronic properties of bulk heterojunction solar cells based on small molecules containing a benzofuran substituted diketopyrrolopyrrole ( DPP ) core (3,6‐bis(5‐(benzofuran‐2‐yl)thiophen‐2‐yl)‐2,5‐bis(2‐ethylhexyl)pyrrolo[3,4‐c]pyrrole‐1,4‐dione, DPP(TBFu)₂ , and [6,6]–phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM) , which were recently reported to have power conversion efficiencies of 4.4%. Electron and hole collection networks are visualized for blends with different donor:acceptor ratios. Formation of nanostructures in the blends leads to a higher interfacial area for charge dissociation, while maintaining bicontinuous collection networks; conditions that lead to the high efficiency observed in the devices. An excellent agreement between nanoscale and bulk open‐circuit voltage measurements is achieved by surface modification of the indium tin oxide (ITO) substrate by using aminopropyltrimethoxysilane. The local open‐circuit voltage is linearly dependent on the cathode work function. These results demonstrate that photoconductive atomic force microscopy coupled with surface modification of ITO substrate can be used to study nanoscale optoelectronic phenomena of organic solar cells.     

Inside Front Cover: Quantitative Measurement of the Local Surface Potential of π‐Conjugated Nanostructures: A Kelvin Probe Force Microscopy Study (Adv. Funct. Mater. 11/2006)

Kelvin probe force microscopy provides quantitative insight into the electronic properties of thin molecular layers, as shown by the results of P. Samorì and co‐workers on p. 1407. In the cartoon shown in the inside front cover, a scanning charged tip probes the local surface potential of a self‐assembled layer, inducing charge polarization into a nanoscale “effective area”. These measurements make it possible to unravel the interplay between structural and electronic properties of molecule‐based materials and devices. We describe a systematic study on the influence of different experimental conditions on the Kelvin probe force microscopy (KPFM) quantitative determination of the local surface potential (SP) of organic semiconducting nanostructures of perylene‐bis‐dicarboximide (PDI) self‐assembled at surfaces. We focus on the effect of the amplitude, frequency, and phase of the oscillating voltage on the absolute surface potential value of a given PDI nanostructure at a surface. Moreover, we investigate the role played by the KPFM measuring mode employed and the tip–sample distance in the surface potential mapping by lift‐mode KPFM. We define the ideal general conditions to obtain a reproducible quantitative estimation of the SP and we find that by decreasing the tip–sample distance, the area of substrate contributing to the recorded SP in a given location of the surface becomes smaller. This leads to an improvement of the lateral resolution, although a more predominant effect of polarization is observed. Thus, quantitative SP measurements of these nanostructures become less reliable and the SP signal is more unstable. We have also devised a semi‐quantitative theoretical model to simulate the KPFM image by taking into account the interplay of the different work functions of tip and nanostructure as well as the nanostructure polarizability. The good agreement between the model and experimental results demonstrates that it is possible to simulate both the change in local SP at increasing tip–sample distances and the 2D potential images obtained on PDI/highly oriented pyrolytic graphite samples. These results are important as they make it possible to gain a quantitative determination of the local surface potential of π‐conjugated nanostructures; thus, they pave the way towards the optimization of the electronic properties of electroactive nanometer‐scale architectures for organic (nano)electronic applications.