Imaging quantum confinement with optical and POWER (perturbations observed with enhanced resolution) NMR

Abstract

The nanoscale distributions of electron density and electric fields in GaAs semiconductor devices are displayed with NMR experiments. The spectra are sensitive to the changes to the nuclear-spin Hamiltonian that are induced by perturbations delivered in synchrony with a line-narrowing pulse sequence. This POWER (perturbations observed with enhanced resolution) method enhanced resolution up to 10 3 -fold, revealing the distribution of perturbations over nuclear sites. Combining this method with optical NMR, we imaged quantum-confined electron density in an individual AlGaAs/GaAs heterojunction via hyperfine shifts. Fits to the coherent evolution and relaxation of nuclei within a hydrogenic state established one-to-one correspondence of radial position to frequency. Further experiments displayed the distribution of photoinduced electric field within the same states via a quadrupolar Stark effect. These unprecedented high-resolution distributions discriminate between competing models for the luminescence and support an excitonic state, perturbed by the interface, as the dominant source of the magnetically modulated luminescence.GaAs ͉ hyperfine or Knight shift ͉ Stark effect ͉ H-band photoluminescence I maging electron distributions and the electrostatic potentials that govern their transfer is a longstanding goal in diverse fields. Spin or charge transfer defines function in systems for photovoltaic and photosynthetic energy capture, ion channels, enzyme catalysis, spintronics, and molecular and nanoscale electronics. Success in imaging nanoscale electronic properties can lead to mechanistic understanding and provide guidelines to tune device performance or to modify natural systems for specialized applications. With its atomic-scale capabilities for nondestructive, noninvasive spectroscopy and imaging, NMR seems well suited to such characterization. However, NMR of solids is hampered by broad spectral lines because of static interactions that prevent measurement of small differences between sites that might otherwise reveal local electronic features with atomic detail.Here, we present a general method whereby solid-state NMR surmounts this challenge and then apply it to image distributions of spin density and electric (E) field within an electronic state with dimensions (Ϸ10 nm) distributed over Ϸ10 5 nuclei. The POWER (perturbations observed with enhanced resolution) NMR approach, encodes small responses to a sample perturbation that is switched in synchrony with an NMR multiple-pulse line-narrowing sequence. The sequence removes the otherwise obscuring clutter of static spin interactions to yield a spectrum dominated by the desired perturbation with up to 3 orders-ofmagnitude resolution enhancement. To further obtain the sensitivity and selectivity needed to isolate signals of local, nanoscale features from the bulk signal of a macroscopic sample, we combined the POWER approach with methods for optical NMR (ONMR). By using both optical nuclear polarization (ONP) (1-15) and optical detection (2-4, 6, 7, 9-13), O...

Nuclear spins in nanostructures

“…Newfound understanding in methods of generating large sustained nuclear polarization, coupled with knowledge of the dissipation of this polarization may help significantly advance the field of biomedical imaging using nuclear magnetic resonance (NMR), which often relies on a large nuclear polarization to enhance sensitivity 33. Finally, a careful analysis of electron–nuclear interactions as well as dynamic polarization in nanostructures can reveal new insights in strongly correlated electron systems, through the use of innovative NMR techniques 34–41.…”

Enhancing time-suspension sequences for the measurement of weak perturbations

“…Similar empirical protocols are noted in the early literature [1]. We emphasize (a) that they should be employed in a context as close as possible to the desired measurement scheme (i.e., not only with the bare sequence, but also incorporating switched interactions), and (b) that the impact of flip angle on the observed quantity (be it a frequency offset [7][8][9]13] or splitting [15,16]) should be evaluated as part of the measurement calibration.…”

“…More recently, the concept has been extended to include imaging switched perturbations to the sample. For example, hyperfine or quadrupolar Stark effects (QSEs) introduced by MPLNsynchronized optical excitation of localized electrons [13,14], or QSEs in response to similarly synchronized external electric fields [15][16][17].…”

Quantitative calibration of radiofrequency NMR Stark effects

“…3,4 More recently, E fields in semiconductors were monitored as direct spectral changes in NMR via quadrupolar Stark effects (QSEs). 5,6 This provided an atomically detailed model of the single-electron charge distribution about a point defect in GaAs, as well as variation of the interfacial E field in a heterojunction. That application, and an earlier proposal for high-resolution quantitation of QSEs, 7 highlights the broader potential of NMR in the arena of electrostatics.…”