N. S. Dellas

Near‐Band‐Edge Electroluminescence from Heavy‐Metal‐Free Colloidal Quantum Dots

The band-edge electroluminescence (EL) of colloidal QDs of cadmium compounds, i.e., Cd(S,Se,Te), exhibits sizetunable spectral emission (450–760 nm) and narrow bandwidth (FWHM ∼ 30–40 nm), allowing for the design and fabrication of color-saturated red, green and blue (RGB) QD-LEDs with simple device confi gurations and high spectral purities that outperform those of liquid crystal displays and organic light emitting diodes. The maximum brightness of RGB QD-LEDs has improved by three orders of magnitude in less than one decade and exceeds 15 000 cd/m 2 for red emitters, corresponding to a current effi ciency of ∼ 2.3 cd/A. [ 15 ] With mercury and lead saltsbased colloidal nanocrystals (HgTe and Pb(S,Se)), the emission

Colloidal nanocrystal-based light-emitting diodes fabricated on plastic toward flexible quantum dot optoelectronics

We report the first demonstration of mechanically flexible quantum dot light-emitting-diodes (QD-LEDs) of all three RGB primary colors. The efficiencies of the flexible devices are high, suggesting the intrinsic flexibility of the QD-based optoelectronic devices.

Orientation dependence of nickel silicide formation in contacts to silicon nanowires

The orientation dependence of Ni silicide phase formation in the silicidation of silicon nanowires (SiNWs) by Ni has been studied. SiNWs with a [112] growth direction contacted by Ni pads form θ-Ni2Si for annealing conditions from 350 to 700 °C for 2 min. The θ-Ni2Si has an epitaxial orientation of θ-Ni2Si[001]∥Si[111¯] and θ-Ni2Si(100)∥Si(112) with the SiNW. On the other hand, SiNWs with a [111] growth direction react with Ni pads to form NiSi2 with an epitaxial orientation of NiSi2[11¯0]∥Si[11¯0] and NiSi2(111)∥Si(111) after annealing at 450 °C for 2 min. The [111] SiNWs were also silicided at 700 °C for 2 min, forming the low-resistivity NiSi phase. The epitaxial phases identified in the reactions of Ni films with SiNWs suggest that lattice matching at both the silicide/Si growth front and the surface of the original SiNW may play a significant role in determining the first silicide segment to grow.The orientation dependence of Ni silicide phase formation in the silicidation of silicon nanowires (SiNWs) by Ni has been studied. SiNWs with a [112] growth direction contacted by Ni pads form θ-Ni2Si for annealing conditions from 350 to 700 °C for 2 min. The θ-Ni2Si has an epitaxial orientation of θ-Ni2Si[001]∥Si[111¯] and θ-Ni2Si(100)∥Si(112) with the SiNW. On the other hand, SiNWs with a [111] growth direction react with Ni pads to form NiSi2 with an epitaxial orientation of NiSi2[11¯0]∥Si[11¯0] and NiSi2(111)∥Si(111) after annealing at 450 °C for 2 min. The [111] SiNWs were also silicided at 700 °C for 2 min, forming the low-resistivity NiSi phase. The epitaxial phases identified in the reactions of Ni films with SiNWs suggest that lattice matching at both the silicide/Si growth front and the surface of the original SiNW may play a significant role in determining the first silicide segment to grow.

Lasing from colloidal InP/ZnS quantum dots

High-quality InP/ZnS core-shell nanocrystal quantum dots (NQDs) were synthesized as a heavy-metal-free alternative to the gain media of cadmium-based colloidal nanoparticles. Upon UV excitation, amplified spontaneous emission (ASE) and optical gain were observed, for the first time, in close-packed InP/ZnS core-shell NQDs. The ASE wavelength can be selected by tailoring the nanocrystal size over a broad range of the spectrum. Moreover, the optical gain profile of InP/ZnS NQDs was matched to the second order feedback of holographic polymer-dispersed liquid crystal gratings, leading to the very first demonstration of an optically-pumped, nanocrystal laser based on InP/ZnS core-shell NQDs.

Nickel and nickel silicide Schottky barrier contacts to n-type silicon nanowires

Schottky contacts to n-type silicon nanowires were fabricated using Ni or nickel silicide contacts in a wraparound or end contact geometry, respectively. Series resistance in the test structures was reduced by heavily doping the opposite end of the silicon nanowire, facilitating Ohmic contact formation and reducing the resistance of the nanowire itself. The effective Schottky barrier height is reported as a function of nanowire doping, ambient, and applied back gate bias, highlighting some of the important variables affecting current transport in Schottky contacts to semiconductor nanowires. For the silicide contact to the most lightly doped silicon nanowire, measurements in N2 showed that the effective barrier height without a back gate bias was 0.69 eV, and the ideality factor was 1.1.

Formation of nickel germanide contacts to Ge nanowires

Nickel germanide contacts are expected to play an important role in Ge-based electronics similar to that of their nickel silicide counterparts in Si devices. Here we have studied the solid state reaction between Ni contact pads and Ge nanowires. We observe the formation of axial nickel germanide segments after annealing at temperatures as low as 300 °C for 2 min. The nickel germanide segments are polycrystalline, without an epitaxial relationship to the Ge nanowire, in contrast to observations of epitaxial nickel silicide formation from Si nanowires. The crystal structure of the nickel germanide phase is consistent with the Ni2In prototype structure. Annealing above 400 °C results in fracture in the nickel germanide segment; however, nickel germanide segments as long as 1.7 μm can be formed by annealing at 400 °C for 5 min.

Electron microscopy of GaAs/MnAs core/shell nanowires

GaAs/MnAs core/shell nanowire heterostructures were synthesized by catalyst-free molecular beam epitaxy. Transmission electron microscopy (TEM) reveals that the GaAs core predominantly grows with the zinc-blende crystal structure with a [111] growth direction. In a small population of wires, the crystal structure transitions from zinc blende to wurtzite with a [001] growth direction. Cross-sectional TEM shows that the MnAs grows epitaxially on the GaAs core in the NiAs prototype structure with an epitaxial relation of [202¯1] MnAs∥[111]GaAs and (011¯0) MnAs∥GaAs (1¯10). When the GaAs core is in the wurtzite structure, the epitaxial relation between the GaAs and MnAs changes to [0001] MnAs∥[0001]GaAs and (1¯21¯0) MnAs∥(1¯21¯0)GaAs.

Measurement and simulation of anisotropic magnetoresistance in single GaAs/MnAs core/shell nanowires

We report four probe measurements of the low field magnetoresistance (MR) in single core/shell GaAs/MnAs nanowires (NWs) synthesized by molecular beam epitaxy, demonstrating clear signatures of anisotropic magnetoresistance that track the field-dependent magnetization. A comparison with micromagnetic simulations reveals that the principal characteristics of the magnetoresistance data can be unambiguously attributed to the nanowire segments with a zinc blende GaAs core. The direct correlation between magnetoresistance, magnetization, and crystal structure provides a powerful means of characterizing individual hybrid ferromagnet/semiconductor nanostructures.

Lithography-free synthesis of freestanding gold nanoparticle arrays encapsulated within dielectric nanowires

A lithography-free method for producing freestanding one-dimensional gold nanoparticle arrays encapsulated within silicon dioxide nanowires is reported. Silicon nanowires grown by the vapor-liquid-solid technique with diameters ranging from 20 nm to 50 nm were used as the synthesis template. The gold nanoparticle arrays were obtained by coating the surface of the silicon nanowires with a 10 nm gold film, followed by thermal oxidation in an oxygen ambient. It was found that the thermal oxidation rate of the silicon nanowires was significantly enhanced by the presence of the gold thin film, which fully converted the silicon into silicon dioxide. The gold-enhanced oxidation process forced the gold into the core of the wire, forming a solid gold nanowire core surrounded by a silicon dioxide shell. Subsequent thermal treatment resulted in the fragmentation of the gold nanowire into a uniformly spaced array of gold nanoparticles encapsulated by a silicon dioxide shell, which was observed by in situ annealing in transmission electron microscopy. Analysis of many different silicon nanowire diameters shows that the diameter and spacing of the gold nanopaticles follows the Rayleigh instability, which confirms this is the mechanism responsible for formation of the nanoparticle array.

Laser-enhanced electroless plating of silver seed layers for selective electroless copper deposition

Copper (Cu) has been patterned using a laser direct-write metallization technique for fabrication of printed wiring boards. The approach consists of writing a silver (Ag) seed layer with a subsequent step of electroless Cu plating to increase thickness and electrical conductivity. Ag seed layers were patterned with a frequency tripled Nd:YVO4 (λ=355 nm) laser with a spot size of approximately 50 μm. Final Cu linewidths after electroless plating were found to be 150 μm with a thickness of 2 μm and electrical resistivity of 5 μΩ cm. An optimal laser power of 0.81 W was found by a factorial-type design experiment. A modern prototype circuit was also patterned with this technique, demonstrating its compatibility with current resolution requirements.