Search

The development of carbon materials as highly efficient and durable substrates for surface-enhanced Raman scattering (SERS) is of great importance for realizing practical application of molecular s...

This study presents a facile and straightforward approach to fabricating periodic arrays of Au nanodisks and vertically aligned, depth- and morphology-controllable Si nanoholes on (001)Si substrates by combining self-assembled nanosphere lithography, selective Au wet etching, and Au nanodisk-catalyzed Si etching processes. All of the etching experiments were carried out in aqueous solutions at room temperature without using complex photolithography and hard-mask patterning processes. The shape, diameter, and spacing of the produced vertical Si nanoholes corresponded well to those of the catalytic Au nanodisks used, and the nanohole depth could be readily tuned from sub-micrometer to several micrometers by adjusting the Au nanodisk-catalyzed Si etching time. The Si substrates with nanohole-textured surfaces exhibited depth-dependent hydrophobic behaviors and broadband (400–1650 nm) light absorption properties. The measured integrated absorptance was found to increase with increasing the depth of Au nanodisk-embedded Si nanoholes and can reach up to 95% in the visible light region and 60% in the near-IR region. The resulting broadband absorption enhancement can be attributed to the combined effects of multiple scattering of light and localized surface plasmon resonance. The obtained results present the exciting prospects that the new combined approach proposed here would offer potential applications in constructing various high-efficiency nanohole array-based optoelectronic and photovoltaic devices.

We report here on the results of a systematic investigation of the interfacial reactions and microstructural evolution of nanoscale Ni metal dots on N 2 + -implanted amorphous Si (a-Si) substrates under different annealing conditions. During annealing, Ni 2 Si was the first phase to form, followed by NiSi and NiSi 2 . The three Ni-silicide phases formed were polycrystalline and the average sizes of the annealed nanodots were observed to increase with the annealing temperature, up to 500 °C. After a further increase of the annealing temperature and/or time, it is interesting to note that the NiSi 2 grains gradually migrated outward from their original nanodot positions to the a-Si regions, which resulted in the formation of a remarkable NiSi 2 nanoring structure. The inner regions of the NiSi 2 nanorings were found to be comprised of a single crystalline Si phase, indicating mediation of the epitaxial crystallization of N 2 + -implanted a-Si by the lateral migration of the NiSi 2 nanodots. Furthermore, the annealing temperature required for complete recrystallization of the a-Si layer in the Ni nanodot/N 2 + -implanted a-Si sample could be significantly reduced to 550 °C, 200 °C lower than that which was required the blank N 2 + -implanted a-Si sample. It is suggested that the formation of these remarkable NiSi 2 nanoring structures and the enhancement of N 2 + -implanted a-Si recrystallization in the presence of NiSi 2 nanodots were due to the silicide-induced crystallization mechanism.

Well-ordered arrays of needle-like and cone-like Si nanostructures have been successfully produced on (001)Si substrates. The morphology and dimensions of the tapered Si nanostructure arrays can be readily controlled by adjusting the Ag-nanoparticle catalytic etching/removal (ACER) cycles. All the produced vertical tapered Si nanostructures were single crystalline and their axial crystallographic orientation was the same as that of the (001)Si substrate. The produced long needle-like Si nanowires exhibited superior field emission properties compared to as-produced and short cone-like Si nanowires. The enhanced field emission performance of the long needle-like Si nanowire array can be attributed to the good vertical alignment, sharp tips, high aspect ratio, single-crystalline structure, and well-ordered arrangement. The experimental results present the exciting prospect that with appropriate control, the combined approach proposed here promises to be applicable for fabricating other highly-ordered, vertically-aligned tapered Si-based nanowire arrays and may offer potential applications in constructing various high efficiency Si-based field emission nanodevices.

In this study, we demonstrated the controllable fabrication of periodic arrays of vertically well-aligned, fully-silicided NiSi2 nanoneedles with sharp nanotips on (100)Si substrates. The new approach proposed here eliminates the need for complicated and costly photolithographic processes and the use of toxic chemicals. All the produced vertical Si and NiSi2 nanoneedles were identified to be single crystalline and with the same geometric morphology and the same axial orientation of [100]. Additionally, the produced tapered NiSi2 nanoneedle array possessed excellent electron emission properties with a very low turn-on field of 0.85 V/μm, which is superior to many reported one-dimensional metal silicide nanostructures. Such an enhancement in the field emission can be attributed to the lower effective work function, sharp nanotips, single-crystalline structure, and good vertical alignment. The combination of the facile approach proposed here and superior electron emission performances make the well-ordered vertical NiSi2 nanoneedles promising candidates for vacuum nanoelectronics and field emission display applications.

We present here the results from a systematic investigation on the growth kinetics and surface properties of Al-doped ZnO (AZO) nanowires synthesized on (0 0 1)Si substrates under different hydrothermal conditions. The as-synthesized vertical AZO nanowires exhibited a hydrophilic characteristic and their crystal structures were determined to be perfectly single crystalline with the axis of the wire parallel to the [0 0 0 1] direction. TEM and EDS results revealed that the as-synthesized AZO nanowires have tapered tips, and the Al-doped concentration in the AZO nanowires was about 1.6 at%. After a series of SEM examinations, the average length of AZO nanowires synthesized at each temperature studied was found to follow a linear relationship with the reaction time, indicating that the hydrothermal growth of AZO nanowires was a reaction-controlled process. The activation energy for linear growth of AZO nanowires on Si substrate, as obtained from an Arrhenius plot, was found to be about 46 kJ/mol. From UV–vis spectroscopic measurements, it was found that the Si substrate coated with vertically-aligned AZO nanowire arrays exhibited remarkably reduced reflectance (10–12%) over a wide range of visible wavelengths (400–800 nm) and angles of light incidence (8–60°). The good broadband and omnidirectional antireflection characteristics can be attributed to the light trapping effect and the graded refractive index resulting from the tapered AZO nanowire structures.

Electromigration in Cu has been extensively investigated as the root cause of typical breakdown failure in Cu interconnects. In this study, Cu nanowires connected to Au electrodes are fabricated and observed using in situ transmission electron microscopy to investigate the electro- and thermo-migration processes that are induced by direct current sweeps. We observe the dynamic evolution of different mass transport mechanisms. A current density on the order of 106 A/cm2 and a temperature of approximately 400 °C are sufficient to induce electro- and thermo-migration, respectively. Observations of the migration processes activated by increasing temperatures indicate that the migration direction of Cu atoms is dependent on the net force from the electric field and electron wind. This work is expected to support future design efforts to improve the robustness of Cu interconnects.

We propose and demonstrate a new and room-temperature approach for the fabrication of well-ordered arrays of vertically aligned, diameter-, length-, and interspacing-controllable Si nanotubes on (0 0 1)Si substrates. In this approach, a unique, hexagonally-ordered Au disk/hole dual nanostructure pattern with uniform size and spacing was first produced on the surface of (0 0 1)Si substrate and was then used as the catalyst to etch vertically downward into the Si substrate by Au-catalyzed chemical etching. All the produced vertical Si nanotubes were identified to be single crystalline with the same axial orientation of [0 0 1] and their lengths could be readily tuned by adjusting the etching time. The produced long vertical Si nanotubes with catalytic Au disk/hole dual nanostructures show greatly enhanced hydrophobicity (water contact angle: ~146°) and broadband absorption properties (average visible-light absorptance: ~96%; average near-IR absorptance: >70%) compared to the corresponding bare (0 0 1)Si substrate and Si nanorod samples. The obtained results present the exciting prospects that the new approach proposed here promises to open opportunities for the design and construction of various 1-D hollow semiconductor nanodevices with multi-function.

Amorphous Si1−xGex films were prepared by co-sputtering on an oxidized Si wafer, followed by rapid thermal annealing to form nanocrystal films. The formation of Ge nanocrystals was not at thermodynamic equilibrium formed in the amorphous Si1−xGex matrix. High-resolution transmission electron microscopy was used to characterize the increase in the size of the grains in the Ge nanocrystals as the Ge content increased. The Ge nanocrystals have a greater absorption in the near-infrared region and higher carrier mobility than SiGe crystals, and the variation in their grain sizes can be used to tune the bandgap. This characteristic was exploited herein to fabricate n-Si1−xGex/p-Si1−xGex p–n diodes on insulating substrates, which were then examined by analyzing their current–voltage characteristics. The rectifying property became stronger as the fraction of Ge in the Si1−xGex films increased. The Si1−xGex diodes are utilized as photodetectors that have a large output current under illumination. This paper elucidates the correlations between the structural, optical and electrical properties and the p–n junction performance of the film.

In this study, we report on the fabrication and characterization of periodic Ni and Ni-silicide nanocontact arrays on (1 1 0)Si substrates. From transmission electron microscopy and selected-area electron diffraction analysis, it is found that the epitaxial NiSi 2 is the first and the only silicide phase formed in the nanoscale Ni contact/(1 1 0)Si sample after annealing at a temperature as low as 300 °C, demonstrating that the nanoscale Ni contact is more favorable for the epitaxial growth of NiSi 2 phase on (1 1 0)Si. The orientation relationship between the epitaxial NiSi 2 nanocontacts and the (1 1 0)Si substrate is identified as [1 1 0]NiSi 2 //[1 1 0]Si and ( 1 ¯ 1 1 ¯ )NiSi 2 //( 1 ¯ 1 1 ¯ )Si. For the samples annealed at higher temperatures, all the epitaxial NiSi 2 nanocontacts formed on (1 1 0)Si are anisotropic in shape and elongated along the crystallographic 1 1 ¯ 0 directions. The observed results can be attributed to the higher surface area to volume ratio of Ni nanocontacts and the faster growth rate along the 〈1 1 0〉 directions than along other directions. The size and periodicity of the nanocontacts can be readily controlled by adjusting the diameter of the colloidal nanosphere template. The self-assembled approach proposed here will provide the capability to fabricate other highly-ordered metal silicide nanocontact arrays and may offer potential applications in constructing silicide-based nanodevices.

The wet cleaning process in semiconductor fabrication often involves the immersion of the copper wafer into etching solutions and thereby its surface properties are significantly altered. The wetting behavior of a copper film deposited on silicon wafer is investigated after a short dip in various etching solutions. The etchants include glacial acetic acid and dilute solutions of nitric acid, hydrofluoric acid, and tetramethylammonium hydroxide. It was found that in most cases a thin oxide layer still remains on the surface of as-received Cu wafers when they are subject to etching treatments. However, a pure Cu wafer can be obtained by the glacial acetic acid treatment and its water contact angle (CA) is about 45°. As the pure Cu wafer is placed in the ambient condition, the oxide thickness grows rapidly to the range of 10–20 A within 3 h and the CA on the hydrophilic surface also rises. In the vacuum, it is surprising to find that the CA and surface roughness of the pure Cu wafer can grow significantly. These interesting results may be attributed to the rearrangement of surface Cu atoms to reduce the surface free energy.

The low temperature sintering of micron-sized Cu powders is achieved by ethanol vapor annealing. A Cu-pancake is formed and has enough mechanical strength to sustain the gravitational pull. The electrical resistivity of the Cu-pancake formed by flaky powders is lower than that by spheroidal ones because more contact area of the former facilitates the sintering process. The resistivity of the Cu-pancake grows with decreasing the annealing temperature but it is still about 10−3 Ω cm at 120 °C. The sintered Cu-pancake is characterized by X-ray diffraction and X-ray photoelectron spectroscopy to investigate the sintering mechanism. The low temperature sintering is attributed to the reduction of the native oxide on surfaces of Cu powders by the ethanol vapor. The reduced Cu is very active and tends to sinter with each other to lower the surface energy. This reduction-assisted sintering may be useful in the fabrication of conductive patterns on flexible substrates. The prepared Cu pattern on polyethylene naphthalate exhibits repeatable flexibility and acceptable conductivity.

We report here the first study of the growth kinetics of vertically-aligned ZnO nanowire arrays grown on Al-doped ZnO (AZO) seed layer-coated substrates by a hydrothermal method. The as-synthesized vertical ZnO nanowires possess a single-crystalline wurtzite structure and a preferred growth orientation along the [0001] direction. The ZnO nanowires were found to grow following the reaction-controlled process and their lengths could be tuned from several to over ten micrometers by adjusting the hydrothermal temperature and time. By measuring the growth rates at different synthesis temperatures, the activation energy for the linear growth of vertical ZnO nanowires on AZO-seeded substrates derived from the slope of the Arrhenius plot was around 35 kJ mol−1. The obtained value is smaller than that of ZnO nanowires grown in bulk solution, which can be explained by the different nucleation mechanisms. From water contact angle measurements, it is found that the as-synthesized ZnO nanowires are hydrophilic in nature, and their surface wettability can be adjusted by the storage time and heat treatment conditions. Furthermore, the reversible switching of the surface wettability of ZnO nanowires has been accomplished by alternate annealing in vacuum and oxygen atmospheres. The ZnO nanowires with switchable surface wettability will find promising applications in surface engineering.

We developed a method to fabricate arrays of nanostructured carbon materials of high quality, via direct pyrolysis of poly(styrene-b-2-vinylpyridine) (PS-b-P2VP) block copolymer (BCP) as thin films without small organic molecules incorporated as carbon resources. Prior to pyrolysis, the nanodomains were cross-linked with UV radiation under nitrogen (UVIN). As upon pyrolysis thermal energy imparts mobility to nanodomains to overcome the constraints imposed by cross-linked chains, the free surface is inevitably covered with the PS block that has a smaller surface energy; unless favorable interactions with the P2VP block exist at the free surface, the outer layer of carbonaceous films is predominantly composed of the pyrolyzed PS block. To overcome this problem, films were capped with a SiO2 layer after UVIN. The capping layer appears to have two advantages – increased areal yields and an improved morphological fidelity. As a result, arrays of nanostructured carbon of great quality were fabricated even at temperatures far above the decomposition temperatures of PS and P2VP blocks.

The pyrolysis of a block copolymer thin film, the free surface of which was in contact with air or a capping layer of SiO2, produced four carbon nanostructures. Thin films of a diblock copolymer having perpendicularly oriented lamellar nanodomains served as carbon and nitrogen precursors. Before pyrolysis, the lamellar nanodomains were cross-linked with UV irradiation under nitrogen gas (UVIN). Without a capping layer, pyrolysis caused a structural transformation from lamellar nanodomains to short carbon nanowires or to dropletlike nanocarbons in a row via Rayleigh instability, depending on the duration of pyrolysis. When capped with a layer of SiO2 followed by pyrolysis, the lamellar nanodomains were converted to pod-like, spaghetti-like, or long worm-like carbon nanostructures. These carbon nanostructures were driven by controlling the surface or interface tension and the residual yield of solid carbonaceous species.

The thin CuO film is acquired by a quick dip of copper in H 2 O 2 solutions at room temperature. The CuO film appears smooth and exhibits superhydrophilic nature. The composition change cannot be verified by X-ray photoelectron spectroscopy but can be manifested by the water contact angle. In the ambient condition, the thickness of the oxidized layer and the surface hydrophobicity grow gradually, while the chemical composition of the overall oxidized film remains essentially unchanged. In the vacuum, the growth rate of the hydrophobicity is significantly elevated, revealing deoxidation on the upmost surface. Our results indicate that growing hydrophobicity on the CuO film is spontaneous and the reversible wettability transition can be observed by H 2 O 2 oxidation and vacuum deoxidation.

In this study, we reported the design, fabrication, and characterization of wellordered arrays of vertically-aligned, epitaxial NiSi2/Si heterostructures and single-crystalline NiSi2 nanowires on (001)Si substrates. The epitaxial NiSi2 with $$\{ \bar 111\}$$ facets was found to be the first and the only silicide phase formed inside the Si nanowires after annealing at a temperature as low as 300 °C. Upon annealing at 500 °C for 4 h, the residual parts of Si nanowires were completely consumed and the NiSi2/Si heterostructured nanowires were transformed to fully silicided NiSi2 nanowires. XRD, TEM and SAED analyses indicated that all the NiSi2 nanowires were single crystalline and their axial orientations were parallel to the [001] direction. The obtained vertically-aligned NiSi2 nanowires, owing to their well-ordered arrangement, single-crystalline structure, and low effective work function, exhibit excellent field-emission properties with a very low turn-on field of 1.1 V/m. The surface wettability of the nanowires was found to switch from hydrophobic to hydrophilic after the formation of NiSi2 phase and the measured water contact angle decreased with increasing extent of Ni silicidation. The increased hydrophilicity can be explained by the Wenzel model. The obtained results present the exciting prospect that the new approach proposed here will provide the capability to fabricate other highly-ordered, vertically-aligned fully silicided nanowire arrays and may offer potential applications in constructing vertical silicide-based nanodevices.

We report here the fabrication of periodic arrays of size- shape-, and spacing-controllable Si nanopores on single-crystalline (110)Si and (111)Si substrates by using our proposed approach, which is based on the self-assembled polystyrene nanosphere lithography in conjunction with the use of oxygen plasma treatments and KOH anisotropic etching processes. Compared with other works, the facile approach proposed here offers a much simpler and low-cost scheme which does not require the use of additional metal-film hard masks deposition and stripping processes. By adjusting the KOH etching duration, the Si nanopore size can be effectively tuned and controlled. The Si nanopores formed on (110)Si and (111)Si were found to be heavily faceted, and their faceted morphologies were mainly determined by the crystal orientations of the Si substrates used. Furthermore, the results of the ultraviolet-visible spectroscopic measurements revealed that the (110)Si substrate with nanopore arrays exhibited significant antireflection properties, and its optical reflectance was found to decrease with increasing the etched nanopore size. The obtained results present the exciting prospects that the combined approach presented here could have significant potential for use in Si-based optoelectronic devices.

The influence of Al addition on the phase transformation and thermal stability of Ni silicides on (0 0 1)Si has been systematically investigated. The presence of Al atoms is found to slow down the Ni 2 Si–NiSi phase transformation but significantly promote the NiSi 2− x Al x formation during annealing. The behavior of phase transformation strongly depends on the Al concentration of the initial Ni 1− x Al x alloys. Compared to the Ni 0.95 Pt 0.05 /Si and Ni 0.95 Al 0.05 /Si system, the Ni 0.91 Al 0.09 /Si sample exhibits remarkably enhanced thermal stability, even after high temperature annealing for 1000 s. The relationship between microstructures, electrical property, and thermal stability of Ni silicides is discussed to elucidate the role of Al during the Ni 1− x Al x alloy silicidation. This work demonstrated that thermally stable Ni 1− x Al x alloy silicides would be a promising candidate as source/drain (S/D) contacts in advanced complementary metal–oxide-semiconductor (CMOS) devices.

Although ammonium nitrate (AN) is a clean burning (chlorine free), non-corrosive, low-hazard, low-flame temperature, inexpensive, easily available energetic material and gives maximum gas horse power per unit weight, propellants incorporating AN suffer from three significant drawbacks: (1) polymorphic (phase) transition near ambient temperature, (2) caking and smoke generation upon burning, and (3) poor flowability for packing. The first two disadvantages could be minimized by forming non-hygroscopic, smokeless 1:1 co-crystals of ammonium nitrate–benzo-18-crown-6 (1:1 co-crystals of AN–B18C6) with a melting point of 125–129 °C, produced from the antisolvent precipitation method based on the systems of: ethanol–methyl-tert-butyl-ether (MTBE), methanol–ethyl acetate and methanol–MTBE. From the smoke test and burning characteristics, 1:1 co-crystals of AN–B18C6 is smokeless upon heating and exhibits a slower burning rate than the one of AN. Therefore, co-crystallization may become a solid-state modification method in the areas of propellants and explosives. The third drawback could further be eliminated by forming spherical agglomerates of 1:1 co-crystals of AN–B18C6 through the addition of water as a bridging liquid into the co-crystal slurry resulted at the end of the antisolvent precipitation method. Spherical agglomerates of 1:1 co-crystals of AN–B18C6 grown from Phase IV AN powders (0.32 mmol) and B18C6 crystals (0.32 mmol) in 0.64 mL methanol + 11.6 mL MTBE + 0.08 mL water gave relatively high yield of 87.3%, high population density of 20.6/cm3, high sphericity, of 0.86, high friability of 100 showing a greater strength, and low angle of repose of 25.2° indicating a good flowability.

The sensors of taste and odor play important roles of recognition as well as reception. In our research, the taste and odor sensing capabilities were based on the photoluminescence (PL) responses of luminescent metal-organic frameworks (MOFs). For the sensing of taste, [In(OH)(bdc)]n(bdc = 1,4-benzenedicarboxylate) and [Tb(btc)] (MOF-76, btc = benzene-1,3,5-tricarboxylate), were tested on aqueous solutions of five basic tastants such as sucrose (sweet), caffeine (bitter), citric acid (sour), sodium chloride (salty) and monosodium glutamate (umami). The photoluminescence (PL) responses of polyacrylic acid-chelated [In(OH)bdc]n and lanthanide Tb(btc) were used to demonstrate the applicability of MOF-based biomimetic tongue through: (1) identification of five tastes: sweet, bitter, sour, salty and umami, by 3-D PCA (principle component analysis) to distinguish the corresponding tastants, (2) quantification of the strength of five tastes determined by the relationships between the PL intensity and the τ scale of taste. For the sensing of odor, [In(OH)(bdc)]nand [Zn4O(bdc)3] (MOF-5) were exposed to the odorants such as cumin, cinnamon, vanillin, p-xylene, m-xylene, o-xylene, water, and ethanol. Similarly, the MOF-based biomimetic nose could distinguish the odors of the analytes based on a pattern recognition method (i.e., principal component analysis) constructed by the 2-D map of PL emission responses.

We present here the results of studies of the synthesis and growth behaviors of electroless pure Co thin films on Pd-activated Si substrates using hydrazine as the reductant. Using the hydrazine-modified electroless Co plating processes, dense and continuous pure Co films were deposited on (0 0 1)Si for samples plated at 30–45 °C. This electroless plating process could be explained by the electrochemical mechanism. After a series of transmission electron microscopic examinations, the deposited Co films were determined to be polycrystalline with a hexagonal crystal structure and the average Co film thickness at each temperature studied was found to follow a linear relationship with the plating time. The deposition rates of pure Co films increase with the plating temperatures from 7.3 nm/min to 12.6 nm/min. By measuring the Co deposition rates at different plating temperatures, the activation energy for linear growth of the electroless Co thin films on Si substrates derived from an Arrhenius plot is about 32.6 kJ/mol. As the plating temperature was increased to 50 °C or higher, the plating solution became turbid and the formation of dendritic cobalt deposits was observed.

We report here on the first study of the interfacial reactions of nanoscale Ni metal dots on single-crystal Si 1− x C x (1 0 0) substrates at various heat treatments. The formation of high-resistivity NiSi 2 phase was found to be more favorable for the miniature size Ni nanodots. The incorporation of C to Si substrates exhibited significant beneficial effects on improving the thermal stability of low-resistivity NiSi nanocontacts. The process window of low-resistivity NiSi in the Ni nanodots/Si 1− x C x (1 0 0) sample was greatly extended by 200–250 °C as compared to that in the Ni nanodots/Si(1 0 0) sample. The presence of C atoms is thought to lower the NiSi nanocontact/Si 1− x C x interface energy and/or to block the Ni diffusion paths during high temperature annealing. For the Ni nanodots/Si 1− x C x (1 0 0) sample annealed at 900 °C, highly curled and tangled amorphous SiO x nanowires with diameters of 8–20 nm were found to form. The growth process of these amorphous SiO x nanowires could be explained by the solid–liquid–solid (SLS) mechanism.

One method was developed for fabricating nanogap structures, which combined the nanosphere lithography, reaction ion etching and glancing deposition technologies. The results show that 10 nm-200 nm nanogaps structure could be prepared by changing the deposition angle, and nanogap structure patterns could be changed with different incident orientation.

In this study, we fabricated well-ordered arrays of site-controlled, vertically-aligned Si nanowires on the desired areas of pre-patterned (001)Si substrates by employing the nanosphere lithographic technique in combination with the Au-assisted selective etching process. The results of transmission electron microscopy and selected-area electron diffraction analysis show that the Si nanowires that fabricated on the patterned (001)Si substrates have a single-crystalline nature and form along the [001] direction. The length of the Si nanowires was found to increase linearly with the Au-assisted etching time. Scanning electron microscopy images clearly revealed that by adjusting the sizes of the nanosphere template and the etching temperature and time, the diameter and length of the patterned Si nanowires could be effectively tuned and accurately controlled. Furthermore, the diameters of the Si nanowires produced at various temperatures and time were found to be relatively uniform over the entire length. The combined approach presented here provides the capability to fabricate a variety of size-, length-tunable 1D Si-based nanostructures on various patterned Si-based substrates.

a b s t r a c t We report here the first successful growth of large-area, low-resistivity cobalt disilicide (CoSi2) nanodot arrays on epitaxial (0 0 1)Si0.7Ge0.3 substrates by using the nanosphere lithography (NSL) technique with an interposing amorphous Si (a-Si) thin film serving as the sacrificial layer. For the Co/a-Si bilayer nanodots array on Si0.7Ge0.3 samples after annealing, polycrystalline CoSi2 appears to form as the only silicide phase at an annealing temperature as low as 500 ◦C The a-Si interlayer with appropriate thickness was found to effectively prevent Ge segregation and maintain the morphological stability in forming CoSi2 nanodots on Si0.7Ge0.3 substrate. The size, interparticle spacing, and triangular shape of the CoSi2 nanodots remain almost unchanged even after annealing at 950 ◦ C. For the Co/a-Si nanodot samples further annealed at

In this study, we demonstrated significant enhancement of the formation of low-resistivity NiSi nanocontacts with controlled size on (0 0 1)Si0.7Ge0.3 substrates by combining the nanosphere lithography with the use of a new Ni/a-Si bilayer nanodot structure. Low-resistivity NiSi with an average size of 78 nm was observed to be the only silicide phase formed in samples annealed at 350–800 °C. The presence of the interposing Si layer with appropriate thickness was found to effectively prevent Ge segregation and maintain the interface stability in forming NiSi nanocontacts on (0 0 1)Si0.7Ge0.3. As the annealing temperature was increased to 900 °C, amorphous SiOx nanowires were observed to grow from silicide nanocontact regions. The NSL technique in conjunction with a sacrificial Si interlayer process promises to be applicable in fabricating periodic arrays of other low-resistivity silicide nanocontacts on Si1−xGex substrates without complex lithography.

As metal-oxide-semiconductor field-effect transistor (MOSFET) devices are shrunk to the nanometer scale, flat shallow metal/Si electrical contacts must be formed in the source/drain region. This work demonstrates a method for the formation of epitaxial NiSi2 layers by a solid-phase reaction in Ni–P(8 nm)/Si(1 0 0) samples. The results show that the sheet resistance remained low when the samples were annealed at temperatures from 400 to 700 °C. P atoms can be regarded as diffusion barriers against the supply of Ni to the Si substrate, which caused the formation of Si-rich silicide (NiSi2) at low temperature. Furthermore, elemental P formed a stable capping layer with O, Ni and Si during the annealing process. A uniform NiSi2 layer with an atomically flat interface was formed by annealing at 700 °C because of the formation of a Si–Ni–P–O capping layer and a reduction in the total interface area.

The annealing effects on the composition distribution of Ge islands on Si (001) were investigated by atomic force microscopy combined with selective wet chemical etching. Experimental results demonstrate that there is a strong composition redistribution occurring during island growth and postgrowth annealing. We observe that, once Ge superdomes appear, the asymmetric composition profile of the Ge domes transforms into an almost symmetric structure. Moreover, the Ge superdomes exhibit a double-ring composition profile of Si after long-time annealing. These phenomena could be explained within a simple thermodynamic model that involves only surface diffusion process.

We report here the first study of formation kinetics of vertically-aligned periodic Si nanorod arrays on (001)Si substrates in H 2 O 2 /HF/EtOH etching solutions. The diameter, length, location, and crystallographic orientation of the Si nanorods produced were well-controlled using the colloidal nanosphere lithography combined with the Au-assisted selective chemical etching process. The as-synthesized Si nanorods were determined to be single crystals and the axial orientation of the Si nanorods was identified to be parallel to the [001] direction, which was identical to the orientation of the (001)Si wafers used. The lengths of Si nanorods could be tuned from sub-micrometer to several micrometers by adjusting the etching temperatures and time. The activation energy for the formation of Si nanorods array on blank-(001)Si was about 76.7 kJ/mole, which was calculated according to the Arrhenius plot. From water contact angle measurements, it is found that the Si substrate with Si nanorod arrays exhibited a more hydrophobic behavior compared to the blank-(001)Si sample. The hydrophobic behavior of the HF-treated Si nanorod arrays could be explained by the Cassie–Baxter model.

article i nfo Available online 13 March 2009 We report here the study on the kinetics and growth of Mg2Ni alloy layers by solid-state reactions in the Mg-Ni system. After appropriate heat treatments, only polycrystalline Mg2Ni phase was found to form on the Mg/Ni interfaces and the grain sizes of Mg2Ni formed near the Mg side were found to be much smaller than those formed near the Ni side. Cross-sectional SEM examinations revealed that the Mg2Ni intermetallic layers grew following the diffusion-control process. During the formation of Mg2Ni layers, the dominant diffusing species was determined to be Ni by using the immobile trench markers. Furthermore, based on the marker analysis, the intrinsic and interdiffusion diffusivities for the Mg-Ni diffusion couples at different temperatures have been determined.

The evolution of the composition distribution and microstructures of Ge islands on Si(001) during the Si overgrowth was investigated by atomic force microscopy combined with selective wet etching procedures. With increasing Si coverage to 5.4 nm, the uncapped Ge islands were found to change their shapes dramatically from domes to truncated pyramids, nanorings and eventually to the fully buried islands. Different atomic composition profiles in SiGe islands were observed at different Si coverages. Especially, the nanorings were found to have a Ge-rich core with a Si-rich periphery. Based on the experimental results, the Ge redistribution in islands during Si capping is not only correlated with the intermixing between Si capping layer and Ge islands, but also a strain-driven process.

In this study we investigate the effects of a thin Au interlayer on the formation of cobalt silicide thin films on (001)Si substrate. The presence of a thin Au interlayer in the Co/Au/Co/(001)Si samples was found to decrease the temperature needed for the formation of the CoSi 2 phase by about 100–190 °C compared to that needed for Co/(001)Si samples. The effect on the formation of CoSi 2 became more pronounced as the thickness of the Au interlayer increased. These results are explained in the context of classical nucleation theory. Cross-sectional transmission electron microscopy (TEM) observations show that both the CoSi 2 surface and CoSi 2 /Si interface in Co/Au/Co/(001)Si samples are much smoother than those in Co/Au/Si samples. In addition, the TEM, energy dispersion spectrometry, and secondary ion mass spectroscopy analyses of the Co/Au/Co/(001)Si samples reveal that after annealing a large amount of Au atoms have diffused from the original interface position to be dispersed in the CoSi 2 layers and at the grain boundaries.

We report here the successful fabrication of large-area size-tunable periodic arrays of cobalt and Co-silicide nanodots on silicon substrates by employing the colloidal nanosphere lithography (NSL) technique and heat treatments. The growth of low-resistivity epitaxial CoSi2 was found to be more favorable for the samples with smaller Co nanodot sizes. The sizes of the epitaxial CoSi2 nanodots can be tuned from 50 to 100 nm by varying the diameter of the colloidal spheres and annealing temperatures. The epitaxial CoSi2 nanodots were found to grow with an epitaxial orientation with respect to the (0 0 1)Si substrates: [0 0 1]CoSi2//[0 0 1]Si and (2 0 0)CoSi2//(4 0 0)Si. From the results of planview HRTEM, XTEM, and SAED analysis, the epitaxial CoSi2 nanodots were identified to be inverse pyramids in shape, and the average sizes of the faceted silicide nanodots were measured to decrease with annealing temperature. The observed results present the exciting prospect that with appropriate controls, the colloidal NSL technique promises to facilitate the growth of a variety of well-ordered silicide nanodots with selected shape, size, and periodicity.

Formation of Co germanosilicides on Si 0.8 Ge 0.2 virtual substrates with a Co/Au/Co sandwich thin film after different heat treatments has been investigated. The sequence of phase formation is the same as the reaction of blanket Co thin film with (001 )Si. The presence of thin interposing Au layers was found to significantly enhance the formation of low-resistivity CoSi 2 on (001) Si 0.8 Ge 0.2 substrates. The formation temperature of CoSi 2 phase in the Co/Au/Co/(001)Si 0.8 Ge 0.2 samples was lowered by about 200°C compared to that of Co/(001 )Si 0.8 Ce 0.2 samples. From TEM and EDS analysis, some of Au atoms were found to diffuse from the original interface position to disperse within the CoSi 2 layers during silicidation reactions. The mechanisms for the enhanced formation of CoSi 2 in the Co/Au/Co/Si 0.8 Ge 0.2 system were explained in the context of classical nucleation theory.

The fabrication of 2-D periodic arrays of nickel metal dots using polystyrene (PS) nanosphere lithography and the interfacial reactions of the Ni dot arrays on (0 0 1) Si substrates after different heat treatments have been investigated. The epitaxial NiSi2 was detected to start growing in samples after annealing at 300 °C. As the annealing temperature was increased to 350–800 °C, only epitaxial NiSi2 nanodot arrays were observed to form. The results revealed that the formation of epitaxial NiSi2 is more favorable for the Ni metal dot arrays samples. The shape of the epitaxial NiSi2 nanodot was found to be inverse pyramidal and the size of the silicide nanodots was measured to diminish with annealing temperature. Furthermore, amorphous SiOx nanowire arrays and single-crystalline Si nanowires were found to form in samples after annealing at 900 and 1100 °C, respectively. The results present the exciting prospect that other nanoscale metal silicide dot and nanowire arrays could be grown on the sub-100 nm pre-patterned Si substrates using the PS nanosphere lithography technique.

Two-dimensional (2D) periodic arrays of Co metal and Co silicide nanodots were successfully fabricated on (0 0 1)Si substrate by using the polystyrene (PS) nanosphere lithography (NSL) technique and thermal annealing. The epitaxial CoSi 2 was found to start growing in samples after annealing at 500 °C. The sizes of the Co silicide nanodots were observed to shrink with annealing temperature. From the analysis of the selected-area electron diffraction (SAED) patterns, the crystallographic relationship between the epitaxial CoSi 2 nanodots and (0 0 1)Si substrates was identified to be [0 0 1]CoSi 2 //[0 0 1]Si and (2 0 0)CoSi 2 //(4 0 0)Si. By combining the planview and cross-sectional TEM examination, the epitaxial CoSi 2 nanodots formed on (0 0 1)Si were found to be heavily faceted and the shape of the faceted epitaxial CoSi 2 nanodot was identified to be inverse pyramidal. The observed results present the exciting prospect that with appropriate controls, the PS NSL technique promises to offer an effective and economical patterning method for the growth of a variety of large-area periodic arrays of uniform metal and silicide nanostructures on different types of silicon substrates.

The fabrications of size-tunable periodic arrays of nickel metal and silicide nanodots on (0 0 1)Si substrates using polystyrene (PS) nanosphere lithography (NSL) and heat treatments have been investigated. The growth of epitaxial NiSi 2 was found to be more favorable for the Ni metal nanodot arrays. The effect becomes more pronounced with a decrease in the size of the Ni nanodots. The sizes of the epitaxial NiSi 2 nanodots were tuned from 38 to 110 nm by varying the diameter of the PS spheres and heat treatment conditions. These epitaxial NiSi 2 nanodots formed on (0 0 1)Si were found to be heavily faceted and the faceted structures were more prone to form at higher temperatures. Based on TEM, HRTEM and SAED analysis, the faceted NiSi 2 nanodots were identified to be inverse pyramids in shape. Compared with the NiSi 2 nanodot arrays formed using single-layer PS sphere masks, the epitaxial NiSi 2 nanodot arrays formed from the double-layer PS sphere templates exhibit larger interparticle spacings and smaller particle sizes. Since the nanoparticle sizes, shapes and interparticle spacings can be adjusted by tuning the diameter of the PS spheres, stacking conditions, and heat treatment conditions, the PS NSL technique promises to be an effective patterning method for growth of other nanostructures.

The interfacial reactions of the 2D-ordered nickel metal nanodots that were prepared by polystyrene nanosphere lithography (NSL) on Si substrates after different heat treatments have been investigated. Epitaxial NiSi2 nanodot arrays were found to form at a temperature as low as 350 °C. The results indicated that the growth of epitaxial NiSi2 is more favorable for the Ni metal dot array samples. The sizes of these epitaxial NiSi2 nanodots in samples annealed at 350–800 °C are in the range of 84–110 nm. The shape of the epitaxial NiSi2 nanodot was found to be pyramidal. Furthermore, for the samples annealed at 900 °C, amorphous SiOx nanowires were found to grow on individual nickel silicide nanoparticles. The diameters of these nanowires are in the range of 15–20 nm. As the size of metal nanodot can be adjusted by tuning the diameter of the polystyrene (PS) spheres, the NSL technique promises to be an effective patterning method without complex lithography.

This study fabricates the optically active uniform SiGe/Si multiple quantum well (MQW) nanorod and nanodot arrays from the Si0.4Ge0.6/Si MQWs using nanosphere lithography (NSL) combined with the reactive ion etching (RIE) process. Compared to the as-grown sample, we observe an obvious blueshift in photoluminescence (PL) spectra for the SiGe/Si MQW nanorod and nanodot arrays, which can be attributed to the transition of PL emission from the upper multiple quantum dot-like SiGe layers to the lower MQWs. A possible mechanism associated with carrier localization is also proposed for the PL enhancement. In addition, the SiGe/Si MQW nanorod arrays are shown to exhibit excellent antireflective characteristics over a wide wavelength range. These results indicate that SiGe/Si MQW nanorod arrays fabricated using NSL combined with RIE would be potentially useful as an optoelectronic material operating in the telecommunication range.

Self-assembled hexagonal Au particle networks, 2–12 μm in cell size, on silicon have been achieved by a simple method. Honeycomb structure of Au nanoparticles on silicon was drop cast from the Au nanoparticle solution under appropriate concentration, evaporation rate, substrate temperature, and humidity. Hexagonal networks with discrete Au particles were generated in samples annealed in N2 ambient. Two-step annealing, i.e., annealing at 400 °C followed by annealing at 1000 °C for 1 h each was found to be effective to improve the regularity of the Au particle network. As the cell size can be adjusted by the tuning of the deposition conditions, the scheme promises to be an effective patterning method without complex lithography.

Ultrafast directional crystallization that combined the electric current stressing with metal-induced crystallization has been achieved for BF2+-implanted amorphous Si (a-Si) at room temperature. Polycrystalline Si was observed to grow from anode towards cathode and the channels of a-Si strips with a length of 140 μm and a width of 10 μm can be fully crystallized with a stressing time less than 0.2 s. The directional growth of crystalline Si nanowires, 50 nm in width and as long as 3 μm in length, with an extraordinarily high aspect ratio of 60, indicates a strong electric-field-induced effect on the growth. The growth method provides a promising scheme to solve the problems caused by high-temperature and long-term annealing treatment for the applications of optoelectronic devices.

The taste sensing capabilities of a “biomimetic tongue” based on the photoluminescence (PL) responses of metal–organic frameworks (MOFs), [In(OH)(bdc)] n (bdc=1,4-benzenedicarboxylate), [Tb(btc)] n (MOF-76, btc=benzene-1,3,5-tricarboxylate), and [Ca 3 (btc) 2 (DMF) 2 (H 2 O) 2 ]·3H 2 O are proven on aqueous solutions of five basic tastants: sucrose (sweet), caffeine (bitter), citric acid (sour), sodium chloride (salty) and monosodium glutamate (umami). For [In(OH)(bdc)] n , the tastant interacts stereochemically with poly(acrylic acid) (PAA) and alters its conformations. The frequency and magnitude of chelation between COO − pendant groups of PAA and In 3+ nodes of [In(OH)(bdc)] n framework influence the corresponding PL reponses. For MOF-76, the tastant interacts with incorporated water in MOF-76 through hydrogen bonding. The limitation of O–H bond stretching of water results in the enhancement of the PL intensity. For [Ca 3 (BTC) 2 (DMF) 2 (H 2 O) 2 ]·3H 2 O, it is added as a third MOF component to increase the precision on taste discrimination. The significance of MOF-based “biomimetic tongue” includes: (1) PAA on [In(OH)(bdc)] n mimics the taste receptor cells (TRCs) for their structural flexibility, (2) the Weber-Fechner law of human sensing that sensation is proportional to the logarithm of the stimulus intensity is observed between the PL emission response of MOF-76 and the concentration of tastant, (3) the strength of taste is quantified by the τ scale and the PL emission intensity of MOF-76, which are dependent on the logarithmic tastant concentration, (4) the tastant is identified by the shape of the 3D principal component analysis contour map (i.e., pattern recognition method), and (5) the fabrication of [In(OH)(bdc)] n /PAA film by brushing is illustrated.

The formation of CoSi and CoSi2 in Si nanowires at 700 and 800 degrees C, respectively, by point contact reactions between nanodots of Co and nanowires of Si have been investigated in situ in a ultrahigh vacuum high-resolution transmission electron microscope. The CoSi2 has undergone an axial epitaxial growth in the Si nanowire and a stepwise growth mode was found. We observed that the stepwise growth occurs repeatedly in the form of an atomic step sweeping across the CoSi2/Si interface. It appears that the growth of a new step or a new silicide layer requires an independent event of nucleation. We are able to resolve the nucleation stage and the growth stage of each layer of the epitaxial growth in video images. In the nucleation stage, the incubation period is measured, which is much longer than the period needed to grow the layer across the silicide/Si interface. So the epitaxial growth consists of a repeating nucleation and a rapid stepwise growth across the epitaxial interface. This is a general behavior of epitaxial growth in nanowires. The axial heterostructure of CoSi2/Si/CoSi2 with sharp epitaxial interfaces has been obtained. A discussion of the kinetics of supply limited and source-limited reaction in nanowire case by point contact reaction is given. The heterostructures are promising as high performance transistors based on intrinsic Si nanowires.

Fully dispersible silicalite nanocrystals have been prepared by a newly developed method that leads to higher than 90% crystalline yield after a short hydrothermal treatment of a transparent semi-crystalline aged precursor sol. A coating sol could be prepared by the proper combination of these nanocrystals, the aged precursor sol and non-ionic surfactant with solvent. The resulted coating on glass substrate showed a broadband antireflection (AR) effect with less than 1% average reflection over the visible range. With proper control of the film thickness, one can shift the reflection minimum to achieve neutral colour. The same type of AR coating sol was further applied to transparent plastic web in a continuous fashion, which will be important to the future flexible display industry.

Large-area, vertically aligned silicon nanowire arrays have been successfully fabricated on (001)Si substrates via the redox reactions between silicon and silver ions in the aqueous solution of AgNO3/HF. From SEM and TEM observations, the typical widths of the synthesized SiNWs are in the range of 30-200 nm. The lengths of SiNWs could be tuned from several to tens of micrometers by adjusting the reaction temperatures and time. For the Ni thin films on Si nanowires samples, the growth of epitaxial NiSi2 was found to be more favorable for the samples with smaller nanowire widths. The epitaxial orientation relationship between the NiSi2 and Si nanowires was identified to be [100]NiSi2// [100]Si and (002)NiSi2//(004)Si. The observed results present the exciting prospect that with appropriate controls, this approach could be applied to investigate the phase transformation and growth kinetics of other metal contacts or interconnects on Si substrates at the nanometer-scale.