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Contact electrochemical replication (CER) is a novel pattern replication methodology advanced in this laboratory that offers the unprecedented capability of direct one-step reproduction of monolayer surface patterns consisting of hydrophilic domains surrounded by a hydrophobic monolayer background (hydrophilic @ hydrophobic monolayer patterns), regardless of how the initial "master" pattern was created. CER is based on the direct electrochemical transfer of information, through aqueous electrolyte bridges acting as an information transfer medium, between two organosilane monolayers self-assembled on smooth silicon wafer surfaces. Upon the application of an appropriate voltage bias between a patterned monolayer/silicon specimen playing the role of "stamp" and a monolayer/silicon specimen playing the role of "target", the hydrophilic features of the stamp are copied onto the hydrophobic surface of the target. It is shown that this electrochemical printing process may be implemented under a variety of experimental configurations conducive to the formation of nanometric electrolyte bridges between stamp and target; however, using plain liquid water for this purpose is, in general, not satisfactory because of the high surface tension, volatility, and incompressibility of water. High-fidelity replication of monolayer patterns with variable size of hydrophilic features was achieved by replacing water with a sponge-like hydrogel that is nonvolatile, compressible, and binds specifically to the hydrophilic features of such patterns. Since any copy resulting from the CER process can equally perform as stamp in a subsequent CER step, this methodology offers the rather unique option of multiple parallel reproduction of an initially fabricated master pattern.
ACS Nano 2008 Dec 23
PMID:Contact electrochemical replication of hydrophilic-hydrophobic monolayer patterns. 1920 92

Polarization anisotropy is investigated in single porous silicon nanoparticles containing multiple chromophores. Two classes of nanoparticles, low current density and high current density, are studied. Low current density samples exhibit red-shifted spectra and contain only one or two chromophores. High current density particles, on average, contain less than four chromophores and display a blue-shifted spectrum. We utilize single-molecule spectroscopy to probe the polarization effects of the particles, and we show that both classes of particles are influenced by a polarized excitation source. These results are exciting at the fundamental level for understanding coupled quantum dot emitters as well as for applications involving single-photon sources or silicon-based polarization-sensitive detectors.
ACS Nano 2008 Jun
PMID:Optical anisotropy in individual porous silicon nanoparticles containing multiple chromophores. 1920 30

Metallic atom-scale junctions (ASJs) constitute the natural limit of nanowires, in which the limiting region of conduction is only a few atoms wide. They are of interest because they exhibit ballistic conduction and their conductance is extraordinarily sensitive to molecular adsorption. However, identifying robust and regenerable mechanisms for their production is a challenge. Gold ASJs have been fabricated electrochemically on silicon using an iodide-containing medium to control the kinetics. Extremely slow electrodeposition or electrodissolution rates were achieved and used to reliably produce ASJs with limiting conductance <5 G(0). Starting from a photolithographically fabricated, Si(3)N(4)-protected micrometer-scale Au bridge between two contact electrodes, a nanometer-scale gap was prepared by focused ion beam milling. The opposing Au faces of this construct were then used in an open-circuit working electrode configuration to produce Au ASJs, either directly or by first overgrowing a thicker Au nanowire and electrothinning it back to an ASJ. Gold ASJs produced by either approach exhibit good stabilityin some cases being stable over hours at 300 Kand quantized conductance properties. The influence of deposition/dissolution potential and supporting electrolyte on the stability of ASJs are considered.
ACS Nano 2008 Aug
PMID:Stable atom-scale junctions on silicon fabricated by kinetically controlled electrochemical deposition and dissolution. 1920 60

"Natural" lithography was used to prepare arrays of nanoscale capacitors on silicon. The capacitance was verified by a novel technique based on the interaction of a charged substrate with the electron beam of a scanning electron microscope. The "nanocapacitors" possessed a capacitance of approximately 1 x 10(-16) F and were observed to hold charge for over an hour. Our results indicate that fabricating nanostructures using natural lithography may provide a viable alternative for future nanoelectronic devices.
ACS Nano 2008 Aug
PMID:Nanocapacitive circuit elements. 1920 63

We investigated hydroxyalkylphosphonate monolayers as a novel platform for the biofunctionalization of silicon-based field effect sensor devices. This included a detailed study of the thin film properties of organophosphonate films on Si substrates using several surface analysis techniques, including AFM, ellipsometry, contact angle, X-ray photoelectron spectroscopy (XPS), X-ray reflectivity, and current-voltage characteristics in electrolyte solution. Our results indicate the formation of a dense monolayer on the native silicon oxide that has excellent passivation properties. The monolayer was biofunctionalized with 12 mer peptide nucleic acid (PNA) receptor molecules in a two-step procedure using the heterobifunctional linker, 3-maleimidopropionic-acid-N-hydroxysuccinimidester. Successful surface modification with the probe PNA was verified by XPS and contact angle measurements, and hybridization with DNA was determined by fluorescence measurements. Finally, the PNA functionalization protocol was translated to 2 microm long, 100 nm wide Si nanowire field effect devices, which were successfully used for label-free DNA/PNA hybridization detection.
ACS Nano 2008 Aug
PMID:Organophosphonate-based PNA-functionalization of silicon nanowires for label-free DNA detection. 1920 69

SiO is the dominant silicon bearing molecule in the circumstellar medium; however, it agglomerates to form oxygen-rich silicates. Here we present a synergistic effort combining experiments in beams with theoretical investigations to examine mechanisms for this oxygen enrichment. The oxygen enrichment may proceed via two processes, namely, (1) chemically driven compositional separation in (SiO)(n) motifs resulting in oxygen-rich and silicon-rich or pure silicon regions, and (2) reaction between Si(n)O(m) clusters leading to oxygen richer and poorer fragments. While SiO(2) molecules are emitted in selected chemical reactions, they readily oxidize larger Si(n)O(n) clusters in exothermic reactions and are not likely to agglomerate into larger (SiO(2))(n) motifs. Theoretically calculated optical absorption and infrared spectra of Si(n)O(m) clusters exhibit features observed in the extended red emissions and blue luminescence from interstellar medium, indicating that the Si(n)O(m) fragments could be contributing to these spectra.
ACS Nano 2008 Aug
PMID:From SiO molecules to silicates in circumstellar space: atomic structures, growth patterns, and optical signatures of SinOm clusters. 1920 78

The fluorescent dye Alexa Fluor 488 or the anticancer drug doxorubicin is attached to the surface and inner pore walls of mesoporous Si particles by covalent attachment, and the oxidation-induced release of each molecule is studied. The molecules are bound to the Si matrix using a 10-undecenoic acid linker, which is attached by thermal hydrosilylation. Loading capacity of the microparticles using this method is approximately 0.5 and 45 mg/g of porous Si microparticle for Alexa Fluor 488 and doxorubicin, respectively. The Si-C-bound assembly is initially stable in aqueous solution, although oxidation of the underlying Si matrix results in conversion to silicon oxide and slow release of the linker-molecule complex by hydrolysis of the Si-O attachment points. When the attached molecule is a fluorophore (Alexa Fluor 488 or doxorubicin), its fluorescence is effectively quenched by the semiconducting silicon matrix. As the particle oxidizes in water, the fluorescence intensity of the attached dye increases due to growth of the insulating silicon oxide layer and, ultimately, dye release from the surface. The recovery of fluorescence in the microparticle and the release of the molecule into solution are monitored in real-time by fluorescence microscopy. Both processes are accelerated by introduction of the oxidizing species peroxynitrite to the aqueous solution. The oxidation-triggered release of the anticancer drug doxorubicin to HeLa cells is demonstrated.
ACS Nano 2008 Nov 25
PMID:Oxidation-triggered release of fluorescent molecules or drugs from mesoporous Si microparticles. 1920 8

Complementary symmetry (CS) Boolean logic utilizes both p- and n-type field-effect transistors (FETs) so that an input logic voltage signal will turn one or more p- or n-type FETs on, while turning an equal number of n- or p-type FETs off. The voltage powering the circuit is prevented from having a direct pathway to ground, making the circuit energy efficient. CS circuits are thus attractive for nanowire logic, although they are challenging to implement. CS logic requires a relatively large number of FETs per logic gate, the output logic levels must be fully restored to the input logic voltage level, and the logic gates must exhibit high gain and robust noise margins. We report on CS logic circuits constructed from arrays of 16 nm wide silicon nanowires. Gates up to a complexity of an XOR gate (6 p-FETs and 6 n-FETs) containing multiple nanowires per transistor exhibit signal restoration and can drive other logic gates, implying that large scale logic can be implemented using nanowires. In silico modeling of CS inverters, using experimentally derived look-up tables of individual FET properties, is utilized to provide feedback for optimizing the device fabrication process. Based upon this feedback, CS inverters with a gain approaching 50 and robust noise margins are demonstrated. Single nanowire-based logic gates are also demonstrated, but are found to exhibit significant device-to-device fluctuations.
ACS Nano 2008 Sep 23
PMID:Complementary symmetry nanowire logic circuits: experimental demonstrations and in silico optimizations. 1920 17

Lithographically patterned nanowire electrodeposition (LPNE) is a new method for fabricating polycrystalline metal nanowires using electrodeposition. In LPNE, a sacrificial metal (M(1)=silver or nickel) layer, 5-100 nm in thickness, is first vapor deposited onto a glass, oxidized silicon, or Kapton polymer film. A (+) photoresist (PR) layer is then deposited, photopatterned, and the exposed Ag or Ni is removed by wet etching. The etching duration is adjusted to produce an undercut approximately 300 nm in width at the edges of the exposed PR. This undercut produces a horizontal trench with a precisely defined height equal to the thickness of the M(1) layer. Within this trench, a nanowire of metal M(2) is electrodeposited (M(2)=gold, platinum, palladium, or bismuth). Finally the PR layer and M(1) layer are removed. The nanowire height and width can be independently controlled down to minimum dimensions of 5 nm (h) and 11 nm (w), for example, in the case of platinum. These nanowires can be 1 cm in total length. We measure the temperature-dependent resistance of 100 microm sections of Au and Pd wires in order to estimate an electrical grain size for comparison with measurements by X-ray diffraction and transmission electron microscopy. Nanowire arrays can be postpatterned to produce two-dimensional arrays of nanorods. Nanowire patterns can also be overlaid one on top of another by repeating the LPNE process twice in succession to produce, for example, arrays of low-impedance, nanowire-nanowire junctions.
ACS Nano 2008 Sep 23
PMID:Lithographically patterned nanowire electrodeposition: a method for patterning electrically continuous metal nanowires on dielectrics. 1920 35

Zinc oxide is a unique material that exhibits exceptional semiconducting, piezoelectric, and pyroelectric properties. Nanostructures of ZnO are equally as important as carbon nanotubes and silicon nanowires for nanotechnology and have great potential applications in nanoelectronics, optoelectronics, sensors, field emission, light-emitting diodes, photocatalysis, nanogenerators, and nanopiezotronics. Fundamental understanding about the growth of ZnO nanowires is of critical importance for controlling their size, composition, structure, and corresponding physical and chemical properties. The papers by She et al. and Ito et al. in this issue describe the controlled growth and field-emission properties of individual nanostructures, respectively. These studies provide new approaches and insight into the controlled growth and electrical properties of ZnO nanostructures.
ACS Nano 2008 Oct 28
PMID:Splendid one-dimensional nanostructures of zinc oxide: a new nanomaterial family for nanotechnology. 1920 46


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