Microwave-enhanced reaction rates for nanoparticle synthesis

Microwave reactor methodologies are unique in their ability to be scaled-up without suffering thermal gradient effects, providing a potentially industrially important improvement in nanocrystal synthetic methodology over convective methods. Synthesis of high-quality, near monodispersity nanoscale InGaP, InP, and CdSe have been prepared via direct microwave heating of the molecular precursors rather than convective heating of the solvent. Microwave dielectric heating not only enhances the rate of formation, it also enhances the material quality and size distributions. The reaction rates are influenced by the microwave field and by additives. The final quality of the microwave-generated materials depends on the reactant choice, the applied power, the reaction time, and temperature. CdSe nanocrystals prepared in the presence of a strong microwave absorber exhibit sharp excitonic features and a QY of 68% for microwave-grown materials. InGaP and InP are rapidly formed at 280 degrees C in minutes, yielding clean reactions and monodisperse size distributions that require no size-selective precipitation and result in the highest out of batch quantum efficiency reported to date of 15% prior to chemical etching. The use of microwave (MW) methodology is readily scalable to larger reaction volumes, allows faster reaction times, removes the need for high-temperature injection, and suggests a specific microwave effect may be present in these reactions.     

Synthesis,spectroscopic characterization and crystal and molecular structure of HRu3(OCN(CH3)2)(CO)10

Dimethylamine reacts with Ru₃(CO)₁₂ to produce the η²-hydrido-η-formamido cluster complex HRu(OCN(CH₃)₂)(CO)₁₀ (I). This formulation is consistent with spectroscopic features such as the absence of v(NH) in the infrared, the presence in the Raman of v(RuHRu) at 1400 cm^?1 (v(RuDRu) at 990 cm^?1) and indication in the ¹H NMR of diastereotopic methyl groups bonded to the nitrogen atom. Since these data could not lead to an unequivocal structure assignment a single crystal X-ray study at 115 K was undertaken. The complex crystallizes in the triclinic space group, P$\text{1}$ with cell dimensions; a 7.299(33) », b 9.5037(40) », c 13.7454(57) », α 91.876(34)°, β 96.387(34)°, γ 95.341(34)° and Z = 2. The structure was solved by a combination of Patterson and Fourier techniques and refined by full matrix least squares to a final R = 0.054 and R_ω = 0.074 for 3074 unique reflections. The three ruthenium atoms define a triangle of unequal sides with both the hydride and formamido groups bridging the longest edge; the formamido group is coordinated through the carbon and oxygen atoms. The edge of the ruthenium triangle bridged both by the hydrogen atom and the formamido group is 2.8755(15) »; the other two edges of the ruthenium triangle are observed to be 2.8319(15) and 2.8577(14) », respectively. In the formamido group the distance CO 1.287(9) » and CN 1.340(10) » reflect partial double bond charater in each bond consistent with observation of two chemically distinct methyl groups on the dinitrogen atom. The hydrogen atom bridging one edge of the ruthenium triangle is asymmetrically positioned at 1.73(9) » from the ruthenium atom bonded to the oxygen atom and 1.91(9) » from the ruthenium atom bonded to the carbon atom of the carboxamido group.     

Electrospray mass spectrometry of

There has been a substantial growth in the application of mass spectrometry (MS) methods for the analysis of inorganic materials, due to the inherent sensitivity of mass spectrometry ionization to the specific composition and structure of the analyzed materials. To date, few mass spectrometry studies have focused on metal-chalcogenide materials, an important class of semiconductor materials at the nanoscale, that exhibit interesting optical and electronic properties as a function of size. In this study, we report the application of a correlated electrospray mass spectrometry (ESMS) study between negative-ion and positive-ion mode under low-cone voltage to probe size, composition, and stability of metal-chalcogenide materials at the <1 nm scale. This correlation approach provides insight into the ionization behavior and thermodynamic stability of clusters in the <1.0 nm size domain of the form [Zn4(SPh)10][Me4N]2, [Cd4(SPh)10][Me4N]2, [E4Zn10(SPh)16][Me4N]4, [E4Cd10(SPh)16][Me4N]4 (E = S, Se). It is demonstrated that application of low-cone voltage ESMS can be a useful technique for the rapid analysis of intact solid state nanomaterials when both negative and positive ionic modes are analyzed, with a potential for extrapolation to other classes of nanoscale materials.     

Fluorescent lifetime quenching near d = 1.5 nm gold nanoparticles: probing NSET validity

The fluorescence behavior of molecular dyes at discrete distances from 1.5 nm diameter gold nanoparticles as a function of distance and energy is investigated. Photoluminescence and luminescence lifetime measurements both demonstrate quenching behavior consistent with 1/d(4) separation distance from dye to the surface of the nanoparticle. In agreement with the model of Persson and Lang, all experimental data show that energy transfer to the metal surface is the dominant quenching mechanism, and the radiative rate is unchanged throughout the experiment.     

Übergangsmetall-Silyl-Komplexe, 44. Darstellung der zweikernigen Silyl-Komplexe (CO)3(R3Si)Fe(μ-PR′R′′)Pt(PPh3)2 durch oxidative Addition von (CO)3(R′R′′HP)Fe(H)SiR3 an (C2H4)Pt(PPh3)2

Transition Metal Silyl Complexes, 44. — Preparation of the Binuclear Silyl Complexes (CO)₃(R₃Si)Fe(μ-PR′R′′)Pt(PPh₃)₂ by Oxidative Addition of (CO)₃(R′R′′HP)Fe(H)SiR₃ to (C₂H₄)Pt(PPh₃)₂ The complexes (CO)₃(R′R′′HP)Fe(H)SiR₃ ( 1 ) [PHR′R′′ = PHPh₂, PH₂Ph, PH₂Cy; SiR₃ = SiPh₃, SiPh₂Me, SiPhMe₂, Si(OMe)₃] react with Pt(C₂H₄)(PPh₃)₂ to give the dinuclear, silyl-substituted complexes (CO)₃(R₃Si)Fe(μ-PR′R′′)Pt(PPh₃)₂ ( 2 ) in high yields. Upon reaction of 2 (R = R′ R′′ = Ph) with CO, the PPh₃ ligand at Pt being trans to the PPh₂ bridge is exchanged, and (CO)₃(Ph₃Si)Fe(μ-PPh₂)Pt(PPh₃)CO ( 3 ) is formed. Complex 3 is characterized by an X-ray structure analysis. The rather short Fe — Si distance [233.9(2) pm] and the infrared spectrum of 3 indicate that the Fe — Pt bond is quite polar.     

NMR analysis of surfaces and interfaces in 2-nm CdSe

Solid-state NMR analysis on wurtzite 2-nm hexadecylamine-capped CdSe nanocrystals (CdSe-HDA) provides evidence of discrete nanoparticle reconstruction within the Se sublattice of the nanomaterial. The cadmium and selenium atoms are probed with (1)H-(113)Cd and (1)H-(77)Se cross-polarization magic angle spinning (MAS) experiments, which demonstrate five ordered selenium sites in the nanoparticle that can be assigned to contributions arising from different surface sites and a selenium site one layer down from the surface. Intriguingly, in these materials both HDA and thiophenol are observed to selectively bind to specific sites on the nanoparticle surface. 2D heteronuclear chemical shift correlation (HETCOR) experiments provide evidence for thiophenol selectively binding at surface vacancies. Analysis of the NMR provides a model of a 2-nm CdSe-HDA molecular surface.     

Molecular "compasses" and "gyroscopes". I. Expedient synthesis and solid state dynamics of an open rotor with a bis(triarylmethyl) frame

We describe our efforts toward the preparation of materials built with molecules possessing topologies analogous to those of macroscopic compasses and gyroscopes. Samples of 1,4-bis(3,3,3-triphenylpropynyl)benzene (3) were prepared by a simple two-step procedure from triphenylmethyl chloride (1) and 1,4-diiodobenzene. The structure of compound 3 is such that the central phenylene can play the role of a gyroscope wheel while the two tritylpropynyl groups can act as an axle and shielding framework. Crystals of a benzene clathrate were characterized by single crystal X-ray diffraction and variable-temperature solid state NMR while their thermal stability was determined by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The rotational dynamics of the phenylene group in the benzene clathrate and in desolvated samples were characterized in terms of a two-fold flipping process. Solid state rotational barriers of ca. 12.8 and 14.6 kcal/mol were estimated for the benzene clathrate and desolvated samples, respectively.     

Enzymatic manipulation of DNA-nanomaterial constructs

The demonstration and control of biofunction between inorganic nanomaterials and biological scaffolding is crucial to the development of the field of biomaterials. Although unique hierarchical structures can be generated, the impact of nanosized materials on the biological activity of DNA-protein interactions is relatively unknown. Using highly selective proteins that induce sequence-specific conformational perturbations within DNA, we demonstrate the absolute maintenance of biofunction for biomaterials composed of duplex DNA appended with 1.4-nm Au particles. Enzyme activity and DNA binding affinities (K(d)) are unaltered by the nanoparticle-DNA conjugates. Our results provide a foundation for interfacing more complex and diverse protein-DNA-systems.     

Involvement of carriers in the size-dependent magnetic exchange for Mn:CdSe quantum dots

The magnetic behavior for Mn:CdSe (0.6%) quantum dots (QDs) exhibits size-dependent magnetic exchange mediated by the concentration of intrinsic carriers, which arise from surface states. High temperature paramagnetic behavior that can be fit to a Brillouin function with weak low temperature antiferromagnetic (AFM) coupling is observed for the large Mn:CdSe (5.0 and 5.8 nm) QDs. The 2.8 and 4.0 nm Mn:CdSe QDs display a size-independent blocking temperature (T(B)) at 12 K, decreasing coercivity with increasing size, and a lowering of the activation barrier for spin relaxation as the QD is increased in size. The magnetic behavior is inconsistent with classical domain theory behavior for a superparamagnet (SPM) but can be accounted for in a carrier-mediated RKKY model. Fitting the susceptibility data reveals a Pauli-paramagnetic (PPM) component that is believed to arise from the presence of carriers. The carrier density is observed to scale with the surface to volume ratio in the QDs, indicating the carriers arise from surface states that are weakly localized resulting in the onset of long-distance carrier-mediated RKKY exchange inducing overall ferrimagnetism in the Mn:CdSe QDs when the carrier concentration is above a critical threshold.