Electrical conduction in materials used in microbolometer technology, such as vanadium oxide (VOx) and amorphous silicon (a-Si), is via carrier hopping between localized states. The hopping conduction parameters determine the temperature coefficient of resistance (TCR), its temperature dependence, and its relationship to resistivity. The electrical noise has a 1/f component that is also associated to the hopping parameters and thus correlated to TCR. Current research on conduction in cross linked metal nanoparticles organized in an insulating matrix shows that TCR and noise can be controlled independently, potentially allowing for precise tailoring of the detector response for differing applications.
Organic spintronics has been an active research field since the demonstration of large magnetoresistance in a thick organic spin valve, in which carrier spins are injected into the organic film sandwiched between two ferromagnetic electrodes. In such spin-injection devices, spin relaxation time and spin diffusion length are key properties that control spin-dependent transport and dictate the device design. Here, we compare spin relaxation and diffusion behaviors in organic solids due to spin-dependent interactions including spin-orbit coupling (SOC), hyperfine interaction (HFI), and exchange. It is found that for SOC-induced spin relaxation, the spin diffusion length is essentially determined by the spin admixture parameter and insensitive to magnetic field and carrier hopping rate, whereas for HFI-induced spin relaxation, it increases rapidly with both the magnetic field and hopping rate. In devices with high-density carriers, where exchange-induced spin motion dominates over carrier hopping, the spin diffusion length is limited by SOC.
We develop a systematic approach of quantifying spin-orbit coupling (SOC) and a rigorous theory of carrier spin
relaxation caused by the SOC in disordered organic solids. The SOC mixes up- and down-spin in the polaron states and
can be characterized by an admixture parameter. The spin relaxation rate is found to be proportional to the carrierhopping
rate, or equivalently, carrier mobility. The spin diffusion length depends on the spin mixing and hopping
distance but is insensitive to the carrier mobility. The SOCs in tris-(8-hydroxyquinoline) aluminum (Alq<sub>3</sub>) and in copper
phthalocyanine (CuPc) are particularly strong, due to the orthogonal arrangement of the three ligands in the former and
Cu 3d orbitals in the latter.The theory quantitatively explains the recent measured spin diffusion lengths in Alq<sub>3</sub> from
muon spin rotation and in CuPc from spin-polarized two-photon photoemission.
We propose core-shell nanorods such as InP-CdS and InP-ZnTe to be photoelectrodes for efficient photoelectrochemical
hydrogen production. Based on our systematic study using strain-dependent <i>k.p</i> theory, we find that in these
heterostructures both energies and wave-function distributions of electrons and holes can be favorably tailored to a
considerable extent by exploiting the interplay between quantum confinement and strain. Consequently, these core-shell
nanorods with proper dimensions (height, core radius, and shell thickness) may simultaneously satisfy all criteria for
effective photoelectrodes in solar-based hydrogen production.