One-dimensional Microwires Research Articles

Morphology evolution and cathodoluminescence properties of non-polar aluminum nitride microwires

The aluminum nitride (AlN) one-dimensional microwires (UWs) are promising candidates for the achievement of highly efficient optoelectronic devices. Here, we report the successful synthetization of large-scale nonpolar AlN UWs without any catalysts using physical vapor transport (PVT). The growth direction of high-quality UWs is along m-orientation. The morphology of AlN UWs was examined by scanning electron microscopy (SEM), and they show a rectangular cross-section with a polyhedral tip. The tip of UW undergoes four morphological evolution driven by growth kinetics and free surface energy minimization. The unique cathodoluminescence (CL) patterns were found in the UW. Both sides of UW exhibit strong violet luminescence (345 nm). In contrast, other violet luminescence (320 nm) and blue luminescence (450 nm) were observed in the center of the UW. The evolution of the morphology of UW crystal suggests that the defect-related luminescence pattern may be related to the difference in impurity incorporation efficiency on a- and m-crystalline planes. The photoluminescence (PL) spectra were obtained and the Fabry–P é rot cavity mode in the microwire was demonstrated. The emission anisotropy of AlN may also contribute to the CL luminescence pattern of the microwires.

Multifunctional properties in both three and one-dimensional polycrystalline Cu-doped Co–Ni-Ga shape memory alloys

The effect of Cu doping on the microstructure and functional properties of polycrystalline Co–Ni-Ga-Cu shape memory alloy is investigated with the Cu composition ranging from 0.3 to 4.5 at.%. For the miniaturization applications in actuators and sensors, the alloy with 3 at.% Cu is selected to fabricate one-dimensional microwires. The results reveal that Cu promotes the growth of the γ phase precipitates, accompanied by the decrease of the martensitic transformation temperature in the bulk Co–Ni-Ga-Cu samples. At low Cu doping, a reversible strain of 4% is obtained. By increasing Cu to 4.5%, a compressive reversible strain can reach at least 2.5% at room temperature. During the compression process, the adiabatic elastocaloric effect of a 3.3 K temperature change is also achieved. Furthermore, the polycrystalline microwire displays a great tensile superelasticity with a larger reversible strain of 5% in a temperature range varying from 223 to 373 K, which is the best among Co–Ni-Ga polycrystalline alloys. The shape memory effect of the microwire with a 3% strain output ranging from 123 K to 353 K is achieved as well. The good multifunctional properties in the Co–Ni-Ga-Cu shape memory alloy bulks and microwires lay a foundation for their applications in sensing, actuating and solid-state refrigeration fields.

Size-Tunable Assembly of Gold Nanoparticles Using Competitive AC Electrokinetics.

Alternating current (AC) electrokinetics is a facile way of patterning colloidal particles into advanced structures. We demonstrate the combined use of AC dielectrophoresis (AC-DEP) and AC electrohydrodynamics (AC-EHD) in a microwell electrode geometry for size-tunable assembly of gold nanoparticles (AuNPs) into one-dimensional microwires and two-dimensional films. The AC-DEP force scales with both particle size and field frequency, whereas the AC-EHD force depends only on the field frequency. So, a critical particle diameter ( dc) exists, below which the EHD phenomenon becomes more important and beyond which the DEP force is dominating. We performed theoretical and experimental studies to determine " dc" and how it gets affected by operating parameters like field frequency, voltage, particle number, electrolyte concentration, electrode size, and geometry. Our results show that the morphologies of the colloidal structures transition from films to microwires as the NP diameters vary from nanometers (< dc) to microns (> dc), and no assembly takes place at intermediate sizes (∼ dc). While the film formation is governed purely by surface EHD flows, microwire synthesis is a result of EHD-assisted DEP phenomenon. Also, a minimum particle number, a low salt concentration, and an optimum frequency range is required to initiate assembly.

Dictating Organolead Perovskite Nanostructure Size Using Silicon Nanotube Templates

A class of alkyl ammonium lead halide structures that readily crystallize as perovskite phases of the generic formula CH3NH3PbX3 have been discovered, and their relative ease of crystallization and formation into robust films have led to the development of an exciting new class of photovoltaic and lasing platforms. To our knowledge, however, synthetic methods that permit an evaluation of the fundamental size dependent properties with the requisite level of control of any of these perovskite structures have not yet been achieved. The key element to achieving feature size control in the synthesis of these perovskite phases is the use of hollow semiconducting silicon nanotubes as templates for perovskite growth; i.e. viewing the nanotube interior as a nanoscale reaction vessel. We have recently reported a facile strategy for the formation of SiNTs of a broadly tunable inner diameter, as well as Si wall thickness. These nanotubes are currently under evaluation for a diverse range of additional uses, such as loading with magnetic nanostructures, drug loading and subsequent delivery, Li storage and discharge phenomena associated with battery technology, etc. In principle, these nanotubes permit structural control of perovskite width through nanotube inner diameter. In this work, we describe the formation of methylammonium lead iodide nanostructures inside porous silicon nanotubes with a wall thickness of 10 nm and possessing inner diameters of either 70 nm or 200 nm. After structural characterization, the photophysical properties of these perovskite nanostructures, in terms of optical absorption and photoluminescence as a function of temperature, are evaluated. Spectroscopic comparisons with relatively larger one-dimensional microwires of the same composition are also carried out. We seek to interrogate not only the presence of size dependent shifts in absorption/emission features associated with a given perovskite structure, but also the effects of physical confinement on possible size-dependent phase behavior of these one dimensional semiconductors.

Controlled Self-Assembly of Organic Microcrystals for Laser Applications

The small organic molecule p-distyrylbenzene (DSB) has been controllably prepared into one-dimensional microwires (1D-MWs) and 2D rhombic microdisks (2D-RMDs) by modulating the growth kinetics in the process of morphology growth. These as-prepared organic microcrystals, 1D-MWs and 2D-RMDs, exhibit a shape-dependent microcavity effect in that the single 1D-MW can act as a Fabry-Perot (FP) mode lasing resonator while the individual 2D-RMD functions as the whispering-gallery-mode (WGM) microcavity. Moreover, as compared with the 1D FP resonators, there exists a higher quality factor (Q) in the WGM lasing resonator under the identical optical path length. Significantly, the lasing threshold, Eth = 1.02 μJ/cm2, of 2D-RMDs is much lower than Eth = 2.57 μJ/cm2 of 1D-MWs. Our demonstration can give the direction for the development of the organic solid-state microlasers.

Electrically driven assembly of CdTe quantum dots into photoconductive microwires

Low energy, low cost quantum dot nanocrystal assembly into one-dimensional microwires for optoelectronic device applications.

Characterization of thermal transport across single-point contact between micro-wires

A contact transient electrothermal technique (CTET) is developed to characterize the thermal transport between one-dimensional conductive and nonconductive microscale wires that are in point contact. This technique is a significant advance from the transient electrothermal method that is used to characterize the thermophysical properties of individual one-dimensional micro-wires. A steady-state analytical solution and a transient numerical solution are used to independently determine the value for the thermal contact resistance between the wires at the contact point. The CTET technique is applied to measurement of the thermal contact resistance between crossed Pt wires (25.4 μm diameter) and the thermal contact resistance between a glass fiber (8.9 μm diameter) in contact with a Pt wire (25.4 μm diameter). For Pt wire contact, the thermal contact resistance increases from 8.94×104 to 7.05×105 K/W when the heating current changes from 20 to 50 mA. For the Pt/glass fiber contact, the thermal contact resistance is much larger (2.83×106 K/W), mainly due to the smaller area at the contact point.