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Brian A Korgel

Publications and source records attributed to Brian A Korgel.

At least 19 recordsLinked to original sources
PubMed2006

Synthesis and magnetic properties of colloidal MnPt3 nanocrystals.

The colloidal synthesis and magnetic properties of MnPt(3) nanocrystals are reported. The nanocrystal size depended on the Mn reactant used, but the Mn:Pt stoichiometry was always 1:3. As synthesized, the nanocrystals are compositionally disordered with the face-centered cubic (fcc) A1 phase. Annealing at 580 degrees C changed the MnPt(3) crystal structure to the compositionally ordered L1(2) phase (AuCu(3) structure) with higher magnetocrystalline anisotropy. Magnetization measurements showed that the A1 nanocrystals are paramagnetic and the L1(2) MnPt(3) nanocrystals are superparamagnetic.

Journal Article↗
PubMed2006

Synthesis and magnetic properties of silica-coated FePt nanocrystals.

Colloidal FePt nanocrystals, 6 nm in diameter, were synthesized and then coated with silica (SiO2) shells. The silica shell thickness could be varied from 10 to 25 nm. As-made FePt@SiO2 nanocrystals have low magnetocrystalline anisotropy due to a compositionally disordered FePt core. When films of FePt@SiO2 particles are annealed under hydrogen at 650 degrees C or above, the FePt core transforms to the compositionally ordered L1(0) phase, and superparamagnetic blocking temperatures exceeding room temperature are obtained. The SiO2 shell prevents FePt coalescence at annealing temperatures up to approximately 850 degrees C. Annealing under air or nitrogen does not induce the FePt phase transition. The silica shell limits magnetic dipole coupling between the FePt nanocrystals; however, low temperature (5 K) and room temperature magnetization scans show slightly constricted hysteresis loops with coercivities that decrease systematically with decreased shell thickness, possibly resulting from differences in magnetic dipole coupling between particles.

Journal Article↗
PubMed2006

Temperature dependence of the field effect mobility of solution-grown germanium nanowires.

The temperature dependence of the field effect mobility was measured for solution-grown single-crystal Ge nanowires. The nanowires were synthesized in hexane from diphenylgermane by the supercritical fluid-liquid-solid process using gold nanocrystals as seeds. The nanowires were chemically treated with isoprene to passivate their surfaces. The electrical properties of individual nanowires were then measured by depositing them on a Si substrate, followed by electrical connection with Pt wires using focused ion beam assisted chemical vapor deposition. The nanowires were positioned over TaN or Au electrodes covered with ZrO2 dielectric that were used as gates to apply external potentials to modulate the conductance. Negative gate potentials increased the Ge nanowire conductance, characteristic of a p-type semiconductor. The temperature-dependent source/drain current-voltage measurements under applied gate potential revealed that the field effect mobility increased with increasing temperature, indicating that the carrier mobility through the nanowire is probably dominated either by a hopping mechanism or by trapped charges in fast surface states.

Journal Article↗
PubMed2006

Breath figure templated self-assembly of porous diblock copolymer films.

Porous polyethylene oxide-b-polyfluorooctylmethacrylate (PEO-b-PFOMA) diblock copolymer films were drop cast onto substrates from Freon (1,1,2-trichlorotrifluoroethane) in a humid atmosphere. The pores in the films exhibit long range hexagonal order in some cases, depending on the PFOMA-to-PEO molecular weight ratio. Films with the best ordered pores were formed with PFOMA-to-PEO ratios of 70 kDa:2 kDa. The pores in the polymer films derive from water droplets that condense as Freon evaporates. The polymer stabilizes the water droplets, or "breath figures," which act as an immiscible template that molds the porous film. Increased polymer hydrophobicity reduces the water wettability of the air/Freon interface, which in turn decreases water droplet nucleation, thus influencing the final pore size and spatial order in the polymer films. We describe how water droplet nucleation influences the final pore size and packing order in the polymer films.

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PubMed2006

Labeling tumor cells with fluorescent nanocrystal-aptamer bioconjugates.

Aptamers that bind to prostate specific membrane antigen (PSMA) were conjugated to luminescent CdSe and CdTe nanocrystals for cell-labeling studies. The aptamer-nanocrystal conjugates showed specific targeting of both fixed and live cells that overexpressed PSMA. More importantly, aptamers were able to label cells dispersed in a collagen gel matrix simulating tissue. The specific binding abilities and synthetic accessibility of aptamers combined with the photostability and small size of semiconductor nanocrystals offers a powerful and general tool for cellular imaging.

Aptamers, Peptide↗
PubMed2006

Self-assembled stripe patterns of CdS nanorods.

Sterically stabilized CdS nanorods, 3-5 nm in diameter with aspect ratios ranging from 4 to 15, were observed to self-align into networks of stripes several micrometers long when drop-cast from dispersions at submonolayer coverage. The stripes are two to three nanorods thick with nanorods assembled side-by-side with their long axes parallel to the stripe direction. Energetic forces between nanorods, such as van der Waals and dipole-dipole attractions, favor parallel nanorod alignment; however, we propose that pattern formation is kinetically limited, or mediated, by solvent evaporation.

Anisotropy↗
PubMed2006

Columnar self-assembly of colloidal nanodisks.

The self-assembly of sterically stabilized colloidal copper sulfide nanodisks, 14-20 nm in diameter and 5-7 nm thick, was studied. The nanodisks were observed by electron microscopy and small-angle X-ray scattering to form columnar arrays when evaporated as thin films from concentrated dispersions. These superstructured nanomaterials might give rise to technologically useful properties, such as anisotropic electrical transport and electrorheological and optical properties.

Colloids↗
PubMed2005

Nickel sulfide and copper sulfide nanocrystal synthesis and polymorphism.

Nickel sulfide and copper sulfide nanocrystals were synthesized by adding elemental sulfur to either dichlorobenzene-solvated (copper sulfide) or oleylamine-solvated metal(II) precursors (nickel sulfide) at relatively high temperature to produce the metal sulfide. Nickel sulfide nanocrystals are cubic Ni(3)S(4) (polydymite) with irregular prismatic shapes, forming by a two-step reduction-sulfidation mechanism where Ni(II) reduces to Ni metal before sulfidation to Ni(3)S(4). Despite extensive efforts to optimize the Ni(3)S(4) nanocrystal size and shape distributions, polydisperse nanocrystals are produced. In contrast, copper sulfide nanocrystals can be obtained with narrow size and shape distributions. The copper sulfide stoichiometry depended on the Cu:S mole ratio used in the reaction: Cu:S mole ratios of 1:2 and 2:1 gave CuS (covellite) and Cu(1.8)S (digenite), respectively. CuS nanocrystals formed as hexagonal disks that assemble into stacked ribbons when cast from solution onto a substrate. CuS, Cu(1.8)S, and Ni(3)S(4) differ from the Cu(2)S and NiS nanocrystals obtained by solventless decomposition of metal thiolate single source precursors, in terms of stoichiometry for copper sulfide, and both stoichiometry and morphology for nickel sulfide [Ghezelbash, A.; Sigman, M. B., Jr.; Korgel, B. A. Nano Lett. 2004, 4, 537-542. Sigman, M. B. Ghezelbash, A.; Hanrath, T.; Saunders, A. E.; Lee, F.; Korgel, B. A. J. Am. Chem. Soc. 2003, 125, 16050-16057].

Journal Article↗
PubMed2005

Strongly birefringent pb3o2cl2 nanobelts.

Orthorhombic Pb3O2Cl2 (mendipite) nanobelts micrometers in length and tens of nanometers wide were synthesized by a solventless thermolysis of a single-source precursor in the presence of capping ligands. The nanobelts are single crystals elongated preferentially in the [010] direction. Pb3O2Cl2 is a birefringent material due to its anisotropic crystal structure. The nanobelts exhibit birefringence enhanced by 1 order of magnitude as a result of their small size and belt geometry exceeding the birefringence of naturally occurring minerals, including CaCO3 and TiO2. The preferential elongation of the nanobelts in the [010] direction contributes to this enhancement.

Journal Article↗
PubMed2005

General shape control of colloidal CdS, CdSe, CdTe quantum rods and quantum rod heterostructures.

We report a general synthetic method for the formation of shape-controlled CdS, CdSe and CdTe nanocrystals and mixed-semiconductor heterostructures. The crystal growth kinetics can be manipulated by changing the injection rate of the chalcogen precursor, allowing the particle shape-spherical or rodlike-to be tuned without changing the underlying chemistry. A single injection of precursor leads to isotropic spherical growth, whereas multiple injections promote epitaxial growth along the length of the c-axis. This method was extended to produce linear type I and type II semiconductor nanocrystal heterostructures.

Journal Article↗
PubMed2005

Influence of surface states on electron transport through intrinsic Ge nanowires.

Solution-grown single-crystal Ge nanowires were used as conductive channels in field effect transistor devices to study the influence of surface states on their electron transport properties. Nanowires contacted with Pt electrodes using focused ion beam metal deposition exhibited linear current-voltage (IV) curves at room temperature with apparent resistivities ranging from 10(1) to 10(-1) Omega cm. In all cases, the nanowire conductance decreased with positive external electric fields applied perpendicular to the nanowire surface by a gate electrode, characteristic of p-type carrier accumulation at the nanowire surface. The field-induced change in conductance exhibited a time-dependent relaxation, with response time and magnitude of current decrease that depended on the nanowire surface chemistry. Nanowires treated with an organic passivation layer using a thermally initiated hydrogermylation reaction exhibited 2 orders of magnitude slower current relaxation and a smaller decrease in current relative to "bare" nanowires with oxidized surfaces.

Journal Article↗