Matthew P. Augustine

Solution reduction synthesis of surface stabilized silicon nanoparticles

This paper describes the preparation of air and moisture stable octanol derivatized crystalline silicon nanoparticles by room temperature sodium naphthalenide reduction of silicon halides.

Low field magnetic resonance images of polarized noble gases obtained with a dc superconducting quantum interference device

Using a low transition temperature superconducting quantum interference device as a detector, we have obtained magnetic resonance images of laser-polarized 3He gas and solid 129Xe at 4.2 K in magnetic fields as low as 0.54 mT (3He) and 1 mT (129Xe), corresponding to Larmor frequencies of 17.6 and 11.8 kHz, respectively. The experimental resolution of the images is ∼500 μm for 3He in the gas phase and ∼950 μm for 129Xe in the solid state.

Comparison of solution-mixed and sequentially processed P3HT:F4TCNQ films: effect of doping-induced aggregation on film morphology

Doping polymeric semiconductors often drastically reduces the solubility of the polymer, leading to difficulties in processing doped films. Here, we compare optical, electrical, and morphological properties of P3HT films doped with F4TCNQ, both from mixed solutions and using sequential solution processing with orthogonal solvents. We demonstrate that sequential doping occurs rapidly (<1 s), and that the film doping level can be precisely controlled by varying the concentration of the doping solution. Furthermore, the choice of sequential doping solvent controls whether dopant anions are included or excluded from polymer crystallites. Atomic force microscopy (AFM) reveals that sequential doping produces significantly more uniform films on the nanoscale than the mixed-solution method. In addition, we show that mixed-solution doping induces the formation of aggregates even at low doping levels, resulting in drastic changes to film morphology. Sequentially coated films show 3–15 times higher conductivities at a given doping level than solution-doped films, with sequentially doped films processed to exclude dopant anions from polymer crystallites showing the highest conductivities. We propose a mechanism for doping induced aggregation in which the shift of the polymer HOMO level upon aggregation couples ionization and solvation energies. To show that the methodology is widely applicable, we demonstrate that several different polymer:dopant systems can be prepared by sequential doping.

Using NMR to study full intact wine bottles.

A nuclear magnetic resonance (NMR) probe and spectrometer capable of investigating full intact wine bottles is described and used to study a series of Cabernet Sauvignons with high resolution 1H NMR spectroscopy. Selected examples of full bottle 13C NMR spectra are also provided. The application of this full bottle NMR method to the measurement of acetic acid content, the detection of complex sugars, phenols, and trace elements in wine is discussed.

General implementation of the ERETIC method for pulsed field gradient probe heads.

A capacitive coupling between a secondary radiofrequency (rf) channel and the gradient coil of a standard commercially available high resolution NMR spectrometer and probe head is described and used to introduce a low level exponentially damped rf signal near the frequency of the primary rf channel to serve as an external concentration standard, in analogy to the so-called ERETIC method. The stability of this inexpensive and simple to implement method, here referred to as the Pulse Into the Gradient (PIG) approach, is superb over a 14-h period and both gradient tailored water suppression and one-dimensional imaging applications are provided. Since the low level signal is introduced via the pulsed field gradient coil, the coupling is identical to that for a free induction signal and thus the method proves to be immune (within 5%) to sample ionic strength effects up to the 2M NaCl solutions explored here.

SQUID detected NMR and NQR

Abstract The dc Superconducting QUantum Interference Device (SQUID) is a sensitive detector of magnetic flux, with a typical flux noise of the order 1 μΦ 0 Hz −1/2 at liquid helium temperatures. Here Φ 0 = ℏ /2 e is the flux quantum. In our NMR or NQR spectrometer, a niobium wire coil wrapped around the sample is coupled to a thin film superconducting coil deposited on the SQUID to form a flux transformer. With this untuned input circuit the SQUID measures the flux, rather than the rate of change of flux, and thus retains its high sensitivity down to arbitrarily low frequencies. This feature is exploited in a cw spectrometer that monitors the change in the static magnetization of a sample induced by radio frequency irradiation. Examples of this technique are the detection of NQR in 27 Al in sapphire and 11 B in boron nitride, and a level crossing technique to enhance the signal of 14 N in peptides. Research is now focused on a SQUID-based spectrometer for pulsed NQR and NMR, which has a bandwidth of 0–5 MHz. This spectrometer is used with spin-echo techniques to measure the NQR longitudinal and transverse relaxation times of 14 N in NH 4 ClO 4 , 63±6 ms and 22±2 ms, respectively. With the aid of two-frequency pulses to excite the 359 kHz and 714 kHz resonances in ruby simultaneously, it is possible to obtain a two-dimensional NQR spectrum. As a third example, the pulsed spectrometer is used to study NMR spectrum of 129 Xe after polarization with optically pumped Rb. The NMR line can be detected at frequencies as low as 200 Hz. At fields below about 2 mT the longitudinal relaxation time saturates at about 2000 s. Two recent experiments in other laboratories have extended these pulsed NMR techniques to higher temperatures and smaller samples. In the first, images were obtained of mineral oil floating on water at room temperature. In the second, a SQUID configured as a thin film gradiometer was used to detect NMR in a 50 μ m particle of 195 Pt at 6 mT and 4.2 K.

Noise triggering of radiation damping from the inverted state

Abstract Spontaneous radiation damping in nuclear magnetic resonance spectroscopy of sharp lines is commonly encountered in highly magnetized samples following magnetization inversion. The time delay for the maximum of the hyperbolic secant like signal to develop following inversion is measured and calculated. The distribution function of the measured delay times is Gaussian–Boltzmann and can be used to predict the sensitivity of both signal delay and phase to inversion errors.

OPTICAL PUMPING MAGNETIC RESONANCE IN HIGH MAGNETIC FIELDS : MEASUREMENT OF HIGH FIELD SPIN EXCHANGE CROSS SECTIONS

Abstract The polarization of 129 Xe gas by spin exchange with optically pumped Rb vapor is examined in high magnetic field. Measurements of the Rb–Xe velocity averaged spin exchange cross section 〈 σ v 〉 and the 129 Xe relaxation time in the absence of Rb, T 1 Xe , are discussed. Comparison of these high field results to low field literature values for 〈 σ v 〉 show that this parameter is essentially field independent. T 1 Xe is also found to be significantly longer in high field. On the basis of these observations, it is shown that production of large amounts of spin polarized 129 Xe gas can be made more efficient by high field operation.

Radiation damping with inhomogeneous broadening: limitations of the single bloch vector model

There are several examples in the literature where effects of inhomoge- neous broadening on radiation damping of free precession signals have been described using a phenomenological 1 / T term in the Bloch equations. The inappropriate use of this 2 model is illustrated for specific cases. Correct signal shapes are predicted only when isochromat solutions of Blochs equations are integrated over the spectrum of the inhomo-

Three component spin echo generation by radiation damping

Exploration of the theory for spin–cavity coupling in inhomogeneously broadened spin ensembles shows that all three components of magnetization can be refocused. The conventional spin echo generated by two components of magnetization is a special case of this three component refocusing in the limit of negligible radiation damping. We demonstrate this effect by an analytical theorem, numerical simulation, and experiment.