Detection and stability of nanoscale space charges in local oxidation nanolithography

We report on the stability of space charges within nanoscale silicon oxide patterns generated by atomic force microscope tip-induced local anodic oxidation of alkyl-terminated silicon. Surface potentials of these structures are investigated using two different approaches: Kelvin probe force microscopy and the spectroscopy of adsorbed charge-sensitive dye molecules. Both techniques prove that there is no decay of the space charge itself at least for several days. The apparent decrease of the surface potential measured with the Kelvin probe method is known to be influenced by the ambient humidity. It is supposed to be caused by a screening effect through the formation of a water layer. This is confirmed by our investigation of the surface potential decrease kinetics, which could be well fitted with an adapted model of water condensation. The fluorescence of the charge-sensitive dye di-4-ANEPPS, which is applied to the structures, shows a spectral shift of about 270 meV compared to an uncharged silicon oxide surface. The high stability of the charges supports the use of local anodic oxidation patterns as templates for selective immobilization of cationic species.     

Fabrication of a one-dimensional array of nanopores horizontally aligned on a Si substrate

A one-dimensional array of nanopores horizontally aligned on a silicon substrate was successfully fabricated by anodic aluminum oxidation (AAO) using a modified two-step procedure. SEM pictures show clear nanostructures of well-aligned one-dimensional nanopore arrays without cracks at the interfaces of the sandwiched structures. The processes are compatible with the planar silicon integrated circuit processing technology, promising for applications in nanoelectronics. The formation mechanism of a single nanopore array on Si substrates was also discussed.     

Fluorescence studies of Rhodamine 6G functionalized silicon oxide nanostructures

Selective anchoring of optically active molecules on nanostructured surfaces is a promising step towards the creation of nanoscale devices with new functionalities. Recently we have demonstrated the electrostatic attachment of charged fluorescent molecules on silicon oxide nanostructures prepared by atomic force microscopy (AFM) nanolithography via local anodic oxidation (LAO) of dodecyl-terminated silicon. In this paper we report on our findings from a more detailed optical investigation of the bound dye Rhodamine 6G. High sensitivity optical wide field microscopy as well as confocal laser microscopy have been used to characterize the Rhodamine fluorescence emission. A highly interesting question concerns the interaction between an emitter close to a silicon surface because mechanisms such as energy transfer and fluorescence quenching will occur which are still not fully understood. Since the oxide thickness can be varied during preparation continuously from 1 to ～ 5 nm, it is possible to investigate the fluorescence of the bound dye in close proximity to the underlying silicon. Using confocal laser microscopy we were also able to obtain optical spectra from the bound molecules. Together with the results from an analysis of their photochemical bleaching behaviour, we conjecture that some of the Rhodamine 6G molecules on the structure are interacting with the oxide, causing a spectral shift and differences in their photochemical properties.     

Physicochemical state of the nanotopographic surface of commercially pure titanium following anodization-hydrothermal treatment reveals significantly improved hydrophilicity and surface energy profiles

A method of coating commercially pure titanium (cpTi) implants with a highly crystalline, thin hydroxyapatite (HA) layer using discharge anodic oxidation followed by hydrothermal treatment (Spark discharged Anodic oxidation treatment ; SA-treated cpTi) has been reported for use in clinical dentistry. We hypothesized that a thin HA layer with high crystallinity and nanostructured anodic titanium oxide film on such SA-treated cpTi implant surfaces might be a crucial function of their surface-specific potential energy. To test this, we analyzed anodic oxide (AO) cpTi and SA-treated cpTi disks by SEM and AFM. Contact angles and surface free energy of each disk surface was measured using FAMAS software. High-magnification SEM and AFM revealed the nanotopographic structure of the anodic titanium oxide film on SA-treated cpTi; however, this was not observed on the AO cpTi surface. The contact angle and surface free energy measurements were also significantly different between AO cpTi and SA-treated cpTi surfaces (Tukey's, P < 0.05). These data indicated that the change of physicochemical properties of an anodic titanium oxide film with HA crystals on an SA-treated cpTi surface may play a key role in the phenomenon of osteoconduction during the process of osseointegration.     

Sub-50 nm positioning of organic compounds onto silicon oxide patterns fabricated by local oxidation nanolithography

We present a process to fabricate molecule-based nanostructures by merging a bottom-up interaction and a top-down nanolithography. Direct nanoscale positioning arises from the attractive electrostatic interactions between the molecules and silicon dioxide nanopatterns. Local oxidation nanolithography is used to fabricate silicon oxide domains with variable gap separations ranging from 40?nm to several microns in length. We demonstrate that an ionic tetrathiafulvalene (TTF) semiconductor can be directed from a macroscopic liquid solution (1?μM) and selectively deposited onto predefined nanoscale regions of a 1?cm(2) silicon chip with?an accuracy of 40?nm.     

Silicon multi-branch nanostructures for decent field emission and excellent electrical transport

We report on the synthesis, field electron emission and electric transport properties of a novel nanomaterial: ordered arrays of crystallized silicon multi-branch nanostructures. A decent field electron emission with relatively low turn-on field of 3.16 V μm?1 and high field-enhancement factor of 1252 was received for the silicon nanobranches. The relevancies between field-emission current-voltage characteristic, turn-on field, threshold field and sample-anode distance have been thoroughly analyzed. In addition, electrical transport measurements revealed a small electrical resistance of 0.51 MΩ for as-prepared silicon nanobranches. In contrast, by improving the silicon nanobranch-electrode contact, vacuum annealing dramatically reduced the electrical resistance, by a factor approaching two, while thermal oxidation resulted in a much higher resistance due to the amorphous oxide coating of the silicon nanobranches, both of the current versus voltage curves became more linear and symmetrical, and the transport stability was obviously improved.     

Humidity sensor structures with thin film porous alumina for on-chip integration

Our aim was to produce a cheap, reliable, low-power and CMOS-MEMS process compatible relative humidity (RH) capacitive sensor that can be incorporated into a state-of-the art wireless sensor network. Porous alumina, produced by electrochemical oxidation of aluminum thin film under anodic bias (AAO, Anodic Aluminum Oxide) was used for the purpose of the sensing layer. We prepared two different capacitive sensor structures on silicon substrate using semiconductor processing steps and anodic oxidation in addition. The first sensor has an ultra-thin, vapor-permeable palladium upper electrode, while in the second case, an electroplated gold-grid is used for the same purpose. The highest achieved average sensitivity is approx. 15 pF/RH%, which is much higher than the values found in product catalogues of discrete, off-the-shelf capacitive humidity sensors (0.2-0.5 pF/RH%).     

Highly ordered hexagonally arranged nanostructures on silicon through a self-assembled silicon-integrated porous anodic alumina masking layer

A combined process of electrochemical formation of self-assembled porous anodic alumina thin films on a Si substrate and Si etching through the pores was used to fabricate ideally ordered nanostructures on the silicon surface with a long-range, two-dimensional arrangement in a hexagonal close-packed lattice. Pore arrangement in the alumina film was achieved without any pre-patterning of the film surface before anodization. Perfect pattern transfer was achieved by an initial dry etching step, followed by wet or electrochemical etching of Si at the pore bottoms. Anisotropic wet etching using tetramethyl ammonium hydroxide (TMAH) solution resulted in pits in the form of inverted pyramids, while electrochemical etching using a hydrofluoric acid (HF) solution resulted in concave nanopits in the form of semi-spheres. Nanopatterns with lateral size in the range 12-200?nm, depth in the range 50-300?nm and periodicity in the range 30-200?nm were achieved either on large Si areas or on pre-selected confined areas on the Si substrate. The pore size and periodicity were tuned by changing the electrolyte for porous anodic alumina formation and the alumina pore widening time. This parallel large-area nanopatterning technique shows significant potential for use in Si technology and devices.     

Positive patterning of ferritin and fibronectin molecules on silicon by the atomic force microscopic anodic oxidation technique

Oxide dots fabricated on silicon (111) by the Atomic Force Microscopy (AFM) anodic oxidation technique was used for the patterning of two different proteins namely, ferritin and fibronectin. Si surfaces were oxidized by the SC1 process and then modified with octadecyltrichlorosilane (OTS) for passivation. Oxide dots were fabricated by applying a bias voltage between the AFM probe and the silicon surface. Furthermore, surface functionalization of oxide dots was achieved through gamma-aminopropyltriethoxysilane (gamma-APTES) and glutaraldehye modification to establish a covalent bond between aldehydes and amino groups of protein molecules. Topographies after each modification steps were monitored by AFM. We were able to achieve positive patterning of ferritin molecules up to an average density of 6 x 10(9)/cm2 on gamma-APTES-covered dots, while 9 x 10(8)/cm2 of ferritin molecules remained on the OTS surface. In contrast to this observation, fibronectin molecules were patterned successfully only on oxide dots, and we did not observe any fibronectin molecules on the OTS surface.     

阳极氧化法制备Ta2O5绝缘膜及性能研究

采用阳极氧化法在纯Ta表面制备绝缘性优良的Ta2O5介质膜,分析阳极氧化制备Ta2O5膜的基本机理,讨论不同电解液、阳极氧化电压及热处理等工艺参数对Ta2O5膜性能的影响.利用XRD、EDS和AFM分析薄膜的组织结构和表面形貌,超高阻微电流测试仪测试Ta2O5绝缘膜漏电流特性和耐击穿电压,结果表明,磷酸电解液中添加适当乙二醇溶液能有效地防止"晶化",阳极氧化电压在125～150V范围内制备Ta2O5绝缘膜耐击穿电压能力强,经350℃/60min大气气氛下热处理Ta2O5薄膜,内部结构致密,能有效提高Ta2O5绝缘膜耐击穿电压.     

Schwingungsverschleiß von ungeschmeierten,faserkeramischen C/C-SiC-Gleitpaarungen

Unlubricated oscillating sliding wear of C/C-Sic fibre reinforced composites C/C-SiC fibre reinforced composites were produced by liquid infiltration technique. Porous C-fibre laminates containing carbonic resin were infiltrated with liquid silicon, leading to a SiC-matrix. Mechanical and thermal properties of the composites were measured. Tribological tests were carried out on self-mated C/C-SiC and C/C-SiC mated with ZrO₂ and steel, respectively, in unlubricated oscillating sliding contact using a ring-on-block tribometer. Environmental conditions such as relative humidity and testing temperature were varied. Microstructures of the composites as well as the worn surfaces were systematically analysed using scanning electron microscopy. Experimental results showed a significant influence of the relative humidity and the testing temperature on tribological properties. Self-lubricating effects due to carbon films occurred at sufficient humidity, contact temperatures < 90°C and below a critical surface pressure.     

Oxidation of the surface of a thin amorphous silicon film

The oxidation of clean crystalline silicon surfaces is self-limiting at moderate oxygen pressures (10^− 5 Pa) and temperatures (500 °C), forming 0.7-0.8 nm thick oxide layers. This study looks at the oxidation of a surface of a thin amorphous silicon film to establish if a similar mechanism is active in this case. We have devised a special experimental procedure to check the oxidation mechanism of thin amorphous silicon films. For the spectroscopic investigations we used photoemission with synchrotron radiation with the highest possible surface sensitivity and resolution. This permits a detailed decomposition of the Si 2p spectral details, using a mathematical decomposition procedure. The results clearly show that the oxidation mechanism of the surface of an amorphous silicon film under similar conditions is severely hindered compared to cases of crystalline substrates, indicating less reactivity at the surface and less transport of oxygen into the amorphous material.     

Direct printing of silver nanoparticles by an agarose stamp on planar and patterned substrates

In this study, we have used an agarose stamp to conduct direct printing of silver nanoparticles, nanowires and nanoplates on both planar and structured substrates. Nanoparticle solution could be first coated on an agarose stamp, and then transferred to a planar substrate. Micro-patterns comprising metal nanoparticles could be printed on planar substrates without the formation of residual layers. Thus a three-dimensional metal microstructure could be easily fabricated. The patterning of electrodes by printing Ag nanowires directly on TiO(2) was also demonstrated to fabricate resistive random access memory (RRAM) devices by all-solution-processing methods. By using a flat agarose stamp, the patterns printed on the microstructured substrates were quite different from those on the nanostructured substrates. On the microstructured substrates, direct printing could print silver nanoparticles onto the protrusion surface, and could print silver layers as thick as several microns, useful for high conductivity electrodes. On the substrates with nanostructures such as photonic crystals or nano-gratings, direct printing could transfer nanoparticles into the grooves or cavities only due to the contact of the agarose stamp with the groove or concavity surface. A new approach to fabricate metal wire grid polarizers was further demonstrated. A nanoporous agarose stamp has a good potential for printing using nanoparticle suspension.     

High speed fabrication of aluminum nanostructures with 10 nm spatial resolution by electrochemical replication

A high fidelity electrochemical replication technique for the rapid fabrication of Al nanostructures with 10?nm lateral resolution has been successfully demonstrated. Aluminum is electrodeposited onto a lithographically patterned Si master using a non-aqueous organic hydride bath of aluminum chloride and lithium aluminum hydride at room temperature. Chemical pretreatment of the Si surface allows a clean detachment of the replicated Al foil from the master, permitting its repetitive use for mass replication. This high throughput technique opens up new possibilities in the fabrication of Al-related nanostructures, including the growth of long range ordered anodic alumina nanochannel arrays.     

Reactive oxygen plasma-enabled synthesis of nanostructured CdO: tailoring nanostructures through plasma-surface interactions

Plasma-assisted synthesis of nanostructures is one of the most precise and effective approaches used in nanodevice fabrication. Here we report on the innovative approach of synthesizing nanostructured cadmium oxide films on Cd substrates using a reactive oxygen plasma-based process. Under certain conditions, the surface morphology features arrays of crystalline CdO nano/micropyramids. These nanostructures grow via unconventional plasma-assisted oxidation of a cadmium foil exposed to inductively coupled plasmas with a narrow range of process parameters. The growth of the CdO pyramidal nanostructures takes place in the solid-liquid-solid phase, with the rates determined by the interaction of plasma-produced oxygen atoms and ions with the surface. It is shown that the size of the pyramidal structures can be effectively controlled by the fluxes of oxygen atoms and ions impinging on the cadmium surface. The unique role of the reactive plasma environment in the controlled synthesis of CdO?nanopyramidal structures is discussed as well.     

Studies on hysteresis reduction in thermally carbonized porous silicon humidity sensor

Different ways to reduce hysteresis in a capacitive-type thermally carbonized porous silicon (TC-PS) humidity sensor are studied and compared. Modification of the contact angle of the dielectric surface, enlargement of the pore size of dielectric, and operating the sensor at elevated temperature proved all to be possible ways to reduce hysteresis in a TC-PS humidity sensor. By variation of the carbonization temperature, we produced TC-PS surfaces of different contact angles. Although the hydrophobic surface prevents hysteresis, it also decreases considerably the sensitivity of the sensor. Enlargement of the pore size reduces and tunes the hysteresis loop into the higher relative humidity (RH) values. Also operation of the sensor only few degrees above room temperature was found to be a workable method to prevent hysteresis. However, a constant temperature is crucial for exact humidity measurement using a TC-PS sensor.     

Local anodic oxidation by atomic force microscopy for nano-Raman strain measurements on silicon-germanium thin films

Nanolithography based on local anodic oxidation (LAO) by atomic force microscopy is a promising technique for patterning strained film nanostructures on the silicon substrates. Due to its versatility and precise control, LAO is suited for preparing well defined calibration structures for local strain measurements. We investigated silicon-germanium patterns prepared by LAO and subsequent selective anisotropic wet etching. By combining the nanolithography and etching, dedicated strain test structures with a line width of 65 nm were achieved and utilized for calibration of tip-enhanced Raman measurements.     

Fabrication and characterization of Si quantum dots and SiO(2) tunnel barriers grown by a controlled oxidation process

The control of the growth of silicon dioxide (SiO(2)) and the formation of quantum dots (QDs) play an important role in the fabrication of single-electron transistors (SETs). In this work, SET?structures were fabricated using a systematic oxidation technique known as the pattern-dependent oxidation (PADOX) process. For comparison, two oxidation processes using conventional furnace and rapid thermal processing (RTP) were used. The oxidation temperature for both oxidation processes was set at 1000?°C and the oxygen flow rate in the furnace was set at 1?l?min(-1). The nanostructures were characterized using AFM, SEM and TEM to determine the quality and the stoichiometry of the Si QDs and the oxides. The oxidation rate using a furnace is 0.36?nm?s(-1), significantly lower than the RTP value which is 2.16?nm?s(-1). Meanwhile, the oxygen contents in SiO(2) grown by furnace and RTP are approximately the?same.     

Preparation of nanostructured silicon oxide and silicon nitride films using nanotechnologies

The synthesis of silicon oxide and silicon nitride surface nanostructures is used as an example to demonstrate the possibility of using correlation relationships with inductive parametric constants for quantitatively assessing the reactivity of surface functional groups and predicting the conditions of surface reactions.     

Water vapor detection with individual tin oxide nanowires

Individual tin oxide nanowires (NWs), contacted to platinum electrodes using focused ion beam assisted nanolithography, were used for detecting water vapor (1500-32?000?ppm) in different gaseous environments. Responses obtained in synthetic air (SA) and nitrogen atmospheres suggested differences in the sensing mechanism, which were related to changes in surface density of the adsorbed oxygen species in the two cases. A model describing the different behaviors has been proposed together with comparative evaluation of NW responses against sensors based on bulk tin oxide. The results obtained on ten?individual devices (tested >6 times) revealed the interfering effect of water in the detection of carbon monoxide and illustrated the intrinsic potential of nanowire-based devices as humidity sensors. Investigations were made on sensitivity, recovery time and device stability as well as surface-humidity interactions. This is the first step towards fundamental understanding of single-crystalline one-dimensional (1D) tin oxide nanostructures for sensor applications, which could lead to integration in real devices.