No abstract

We have used transient terahertz photoconductivity measurements to assess the efficacy of two-temperature growth and core-shell encapsulation techniques on the electronic properties of GaAs nanowires. We demonstrate that two-temperature growth of the GaAs core leads to an almost doubling in charge-carrier mobility and a tripling of carrier lifetime. In addition, overcoating the GaAs core with a larger-bandgap material is shown to reduce the density of surface traps by 82%, thereby enhancing the charge conductivity.Semiconductor nanowires are promising new materials for implementation in nanoscale electronic and optoelectronic devices. Of particular interest are III-V semiconductor nanowires, which can exhibit a direct bandgap and a high electron mobility.1 However, the large surface-to-volume ratio inherent to nanowires results in the presence of surface traps offering easy access to carrier and exciton recombination pathways.2,3 In addition, one-temperature growth techniques have been shown to cause a significant twin-defect (stacking-defect) density within the nanowires. 4 Refinements in the epitaxial growth of these nanowires are therefore essential in order for their optoelectronic and crystallographic standards to approach those of bulk material.2,6,7 Such efforts are complicated by the fact that electrical measurements conducted on nanowires to determine charge-carrier mobility are often obscured by properties of the electrical contacts. Most contactless spectroscopic probes of nanowires to date have relied upon low-temperature photoluminescence measurements to characterize optoelectronic quality by measuring excitonic dynamics and radiative quantum efficiency. 2,6However, for use of these materials in nanoelectronics and optoelectronics, it is essential to determine charge-carrier mobility and lifetime at room temperature.In this study, we have conducted transient photoconductivity measurements on an ensemble of nanowires in order to assess the effect of nearly defect-free (two-temperature) growth and core-shell encapsulation technologies on charge-carrier trapping and mobility. Optical-pump terahertz-probe spectroscopy was employed as a noncontact ultrafast probe of the room-temperature photoconductivity with subpicosecond resolution. We demonstrate that both two-temperature growth and encapsulation of the GaAs nanowires with a higher band gap material lead to significant increases in the lifetime of free charge carriers. Encapsulation of the nanowires is shown to be highly effective, reducing the areal density of surface traps to one-seventh of that for the untreated wires. Importantly, we find that moving from one-temperature growth to a two-temperature procedure (comprising a brief high-temperature step for nucleation and a longer lower-temperature phase for prolonged growth 4 ) increases the intrinsic carrier mobility of the wires from 1200 cm 2 /(V s) to 2250 cm 2 /(V s).All nanowire samples were initially grown onto a GaAs substrate as shown in a representative scanning electron microscopy ...

Singlet exciton fission is the spin-conserving transformation of one spin-singlet exciton into two spin-triplet excitons. This exciton multiplication mechanism offers an attractive route to solar cells that circumvent the single-junction Shockley-Queisser limit. Most theoretical descriptions of singlet fission invoke an intermediate state of a pair of spin-triplet excitons coupled into an overall spinsinglet configuration, but such a state has never been optically observed. In solution, we show that the dynamics of fission are diffusion limited and enable the isolation of an intermediate species. In concentrated solutions of bis(triisopropylsilylethynyl)[TIPS]-tetracene we find rapid (<100 ps) formation of excimers and a slower (∼10 ns) break up of the excimer to two triplet exciton-bearing free molecules. These excimers are spectroscopically distinct from singlet and triplet excitons, yet possess both singlet and triplet characteristics, enabling identification as a triplet pair state. We find that this triplet pair state is significantly stabilized relative to free triplet excitons, and that it plays a critical role in the efficient endothermic singlet fission process.T he fission of photogenerated spin-singlet excitons into pairs of spin-triplet excitons is an effective way to generate triplet excitons in organic materials (1, 2). Because the triplets produced are coupled into an overall singlet state, spin is conserved and triplet formation can proceed on sub-100-fs timescales (1, 3-5) with yields of up to 200% (1, 6, 7). Current interest in singlet fission is driven by its potential to improve the efficiency of solar cells by circumventing the Shockley-Queisser limit for single-junction devices (8-10). Incorporating singlet fission material within a lowband-gap solar cell should make it possible to capture the energy normally lost to thermalization following the absorption of highenergy photons (11,12). An external quantum efficiency of 129% (13) and an internal quantum efficiency of >180% (14) have been reported using pentacene as the singlet fission material and fullerene (C 60 ) as electron acceptor. Despite such significant advances, many questions remain about the underlying mechanism of triplet formation, such as the role of intermediate electronic states and the ability of systems to undergo endothermic fission.The basis of most kinetic descriptions of singlet fission is the triplet pair state 1 (TT), which is entangled into an overall singlet and is an essential intermediate for the formation of two free triplet excitons (1, 15). Whether this intermediate state is present only transiently, as expected in exothermic systems such as pentacene, or whether it can be sufficiently long lived to also play a central role in the fission process in slower systems is unclear. Transient absorption measurements of the canonical systems pentacene and tetracene in the solid state allow clear identification of only the singlet and triplet states (3,6,16,17), meaning the character of any intermediate has not been ob...