D. P. Waters

Robust absolute magnetometry with organic thin-film devices

Magnetic field sensors based on organic thin-film materials have attracted considerable interest in recent years as they can be manufactured at very low cost and on flexible substrates. However, the technological relevance of such magnetoresistive sensors is limited owing to their narrow magnetic field ranges (∼30 mT) and the continuous calibration required to compensate temperature fluctuations and material degradation. Conversely, magnetic resonance (MR)-based sensors, which utilize fundamental physical relationships for extremely precise measurements of fields, are usually large and expensive. Here we demonstrate an organic magnetic resonance-based magnetometer, employing spin-dependent electronic transitions in an organic diode, which combines the low-cost thin-film fabrication and integration properties of organic electronics with the precision of a MR-based sensor. We show that the device never requires calibration, operates over large temperature and magnetic field ranges, is robust against materials degradation and allows for absolute sensitivities of <50 nT Hz−1/2.

Room-temperature coupling between electrical current and nuclear spins in OLEDs

Organic semiconductors go out for a spin Magnetism is a commonly observed phenomenon in the macroscopic world, but its origins lie in the quirky quantum-mechanical property of electrons and certain nuclei known as spin. Recent research has sought to leverage and expand the role of spin in the operation of electronic devices. Malissa et al. used a highly sensitive spectroscopic technique to probe, and ultimately manipulate, the subtle effects of spin interactions on the current that flows through organic light-emitting diodes (OLEDs) (see the Perspective by Bobbert). They pinpointed coupling between the spins of the current carriers and the hydrogen nuclei in the hydrocarbon-based material making up the device. Science, this issue p. 1487; see also p. 1450 Magnetic resonance spectroscopy enables detection and manipulation of subtle spin interactions in organic semiconductors. [Also see Perspective by Bobbert] The effects of external magnetic fields on the electrical conductivity of organic semiconductors have been attributed to hyperfine coupling of the spins of the charge carriers and hydrogen nuclei. We studied this coupling directly by implementation of pulsed electrically detected nuclear magnetic resonance spectroscopy in organic light-emitting diodes (OLEDs). The data revealed a fingerprint of the isotope (protium or deuterium) involved in the coherent spin precession observed in spin-echo envelope modulation. Furthermore, resonant control of the electric current by nuclear spin orientation was achieved with radiofrequency pulses in a double-resonance scheme, implying current control on energy scales one-millionth the magnitude of the thermal energy.

Spin-Relaxation Dynamics of E′ Centers at High Density in SiO2 Thin Films for Single-Spin Tunneling Force Microscopy

Methods for the creation of thin amorphous silicon dioxide (aSiO2) layers on crystalline silicon substrates with very high densities of silicon dangling bonds (so called E’ centers) have been explored and volume densities of [E’]> 5 × 1018cm−3 throughout a 60nm thick film have been demonstrated by exposure of a thermal oxide layer to a low pressure Argon radio frequency plasma. While the generated high E’ center densities can be annealed completely at 300C, they are comparatively stable at room temperature with a half life of about one month. Spin relaxation time measurements of these states between T = 5K and T = 70K show that the phase relaxation time T2 does not strongly depend on temperature and compared to SiO2 films of lower E’ density, is significantly shortened. The longitudinal relaxation time T1 ≈ 195(5)μs at room temperature is in agreement with low–density SiO2. In contrast, T1 ≈ 625(51)μs at T = 5K is much shorter than in films of lower E’ density. These results are discussed in the context of E’ centers being used as probe spins for spin–selection rules based single spin–readout.