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Avalanche photodetectors (APDs) are key components in optical communication systems due to their increased photocurrent gain and short response time as compared to conventional photodetectors. A detector design where the multiplication region is implemented in a large band gap material is desired to avoid detrimental Zener tunneling leakage currents, a concern otherwise in smaller band gap materials required for absorption at 1.3/1.55 μm. Self-assembled III-V semiconductor nanowires offer key advantages such as enhanced absorption due to optical resonance effects, strain-relaxed heterostructures, and compatibility with mainstream silicon technology. Here, we present electrical and optical characteristics of single InP and InP/InAsP nanowire APD structures. Temperature-dependent breakdown characteristics of p+-n-n+ InP nanowire devices were investigated first. A clear trap-induced shift in breakdown voltage was inferred from I-V measurements. An improved contact formation to the p+-InP segment was observed upon annealing, and its effect on breakdown characteristics was investigated. The band gap in the absorption region was subsequently varied from pure InP to InAsP to realize spatially separate absorption and multiplication APDs in heterostructure nanowires. In contrast to the homojunction APDs, no trap-induced shifts were observed for the heterostructure APDs. A gain of 12 was demonstrated for selective optical excitation of the InAsP segment. Additional electron-beam-induced current measurements were carried out to investigate the effect of local excitation along the nanowire on the I-V characteristics. Simulated band profiles and electric field distributions support our interpretation of the experiments. Our results provide important insight for optimization of avalanche photodetector devices based on III-V nanowires. © 2017 American Chemical Society The authors acknowledge financial support from NanoLund, the Swedish Research Council, the Swedish National Board for Industrial and Technological Development, the Swedish Foundation for Strategic Research, the Ljungberg Foundation, the Carl Trygger Foundation, and the Swedish Energy Agency. This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No. 641023 (NanoTandem) and under the Marie Sklodowska-Curie grant agreement No. 656208.

Core-shell nanowires offer great potential to enhance the efficiency of light-emitting diodes (LEDs) and expand the attainable wavelength range of LEDs over the whole visible spectrum. Additionally, nanowire (NW) LEDs can offer both improved light extraction and emission enhancement if the diameter of the wires is not larger than half the emission wavelength (λ/2). However, AlGaInP nanowire LEDs have so far failed to match the high efficiencies of traditional planar technologies, and the parameters limiting the efficiency remain unidentified. In this work, we show by experimental and theoretical studies that the small nanowire dimensions required for efficient light extraction and emission enhancement facilitate significant loss currents, which result in a low efficiency in radial NW LEDs in particular. To this end, we fabricate AlGaInP core-shell nanowire LEDs where the nanowire diameter is roughly equal to λ/2, and we find that both a large loss current and a large contact resistance are present in the samples. To investigate the significant loss current observed in the experiments in more detail, we carry out device simulations accounting for the full 3D nanowire geometry. According to the simulations, the low efficiency of radial AlGaInP nanowire LEDs can be explained by a substantial hole leakage to the outer barrier layer due to the small layer thicknesses and the close proximity of the shell contact. Using further simulations, we propose modifications to the epitaxial structure to eliminate such leakage currents and to increase the efficiency to near unity without sacrificing the λ/2 upper limit of the nanowire diameter. To gain a better insight of the device physics, we introduce an optical output measurement technique to estimate an ideality factor that is only dependent on the quasi-Fermi level separation in the LED. The results show ideality factors in the range of 1-2 around the maximum LED efficiency even in the presence of a very large voltage loss, indicating that the technique is especially attractive for measuring nanowire LEDs at an early stage of development before electrical contacts have been optimized. The presented results and characterization techniques form a basis of how to simultaneously optimize the electrical and optical efficiency of core-shell nanowire LEDs, paving the way to nanowire light emitters that make true use of larger-than-unity Purcell factors and the consequently enhanced spontaneous emission.

Position controlled nanowire growth is important for nanowire-based optoelectronic components which rely on light emission or light absorption. For solar energy harvesting applications, dense arrays of nanowires are needed; however, a major obstacle to obtaining dense nanowire arrays is seed particle displacement and coalescing during the annealing stage prior to nanowire growth. Here, we explore three different strategies to improve pattern preservation of large-area catalyst particle arrays defined by nanoimprint lithography for nanowire growth. First, we see that heat treating the growth substrate prior to nanoimprint lithography improves pattern preservation. Second, we explore the possibility of improving pattern preservation by fixing the seed particles in place prior to annealing by modifying the growth procedure. And third, we show that a SiNx growth mask can fully prevent seed particle displacement. We show how these strategies allow us to greatly improve the pattern fidelity of grown InP nanowire arrays with dimensions suitable for solar cell applications, ultimately achieving 100% pattern preservation over the sampled area. The generic nature of these strategies is supported through the synthesis of GaAs and GaP nanowires. [Figure not available: see fulltext.] (Less)

Time-resolved electron paramagnetic resonance (TREPR) spectroscopy was used for studying photoinduced intramolecular electron transfer (ET) and energy transfer (EnT) processes in three mechanically interlocked molecules in which zinc(II)porphyrin (ZnP) and C60 fullerene moieties are arrayed around a central Cu(I) bisphenanthroline core used to assemble these donor–acceptor (D–A) systems. The specific molecules studied include a “long” (ZnP)2–Cu(I)(phen)2–C60 rotaxane as well as ZnP–Cu(I)(phen)2 and ZnP–Cu(I)(phen)2–C60 catenanes, embedded in different phases of nematic liquid crystal and frozen isotropic solvents. It was demonstrated that the routes and rates of the transfer processes in these supramolecules strongly depend on the characteristics of their microenvironment and the molecular entity which was selectively photoexcited. This is reflected by formation of distinct long-lived charge-separated species such as the radical ion pair (ZnP)2•+–Cu(I)(phen)2•– and the metal-to-ligand charge-separated sta...

The effect of the corrole macrocycle bromination on its photoexcited triplet state parameters was examined in a comparison study of brominated and nonbrominated Ga(III) 5,10,15-tris(pentafluorophenyl)corroles, employing X-band (9.5 GHz) time-resolved electron paramagnetic resonance (TREPR) spectroscopy. It is demonstrated that the spectrum of the brominated Ga-corrole is characterized by an opposite polarization pattern and a larger zero-field splitting (ZFS) parameter |D|, compared to nonbrominated Ga-corrole. With the assignment of a negative sign for the ZFS parameter D, the dominant intersystem crossing (ISC) pathways are evaluated. Spectral line shape analysis reveals that in the brominated Ga-corrole,the out-of-plane triplet sublevel is overpopulated (A(Z) > A(X), A(Y)), while in nonbrominated Ga-corrole, the in-plane triplet sublevels are preferentially populated (A(X), A(Y) ≫ A(Z)). The differences in the photophysical properties of the corroles are attributed to the heavy atom effect upon corrole skeleton bromination, which enhances the spin-orbit coupling strength in the brominated complex, thus affecting its ISC and ZFS parameters.

Photoinduced intramolecular electron-transfer (ET) and energy-transfer (EnT) processes in two rotaxanes, one containing both zinc porphyrin and C(60) fullerene moieties incorporated around the Cu(I) bisphenanthroline core [(ZnP)(2)-Cu(I)(phen)(2)-C(60)] and a second complex lacking the fullerene [(ZnP)(2)-Cu(I)(phen)(2)], were studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy at 9.5 GHz (X-band) combined with a selective photoexcitation of the rotaxane moieties. The experiments were carried out in isotropic toluene and ethanol and in anisotropic nematic liquid-crystal (E-7) media over a wide range of temperatures corresponding to the different states of the solvents. The TREPR results are compared with those obtained previously by optical methods in dichloromethane at room temperature. It is demonstrated that the efficiencies and pathways of the light-driven ET and EnT processes in both rotaxanes strongly depend on the properties of their microenvironment, resulting in the formation of distinct charge-separated states under different experimental conditions. The complementary results revealed by the optical and TREPR techniques are attributed to the relatively high conformational mobility of the mechanically interlocked rotaxane systems. Because of the solute-solvent interactions, the rotaxanes are able to change conformation in different microenvironments, which affects the parameters of the photoinduced processes occurring in these systems.

Three photosensitizers based on tris-(pentafluorophenyl)antimony corroles that differ in oxidation state and axial ligands, namely, (pyridine) Sb (III)-, (oxo) Sb (V)- and (difluoro) Sb (V) complexes, were studied by time-resolved electron paramagnetic resonance spectroscopy and laser flash photolysis. The magnetic and orientational parameters of the corroles oriented in a nematic liquid crystal as well as their triplet lifetimes in liquid toluene were determined and interpreted in terms of their structure and geometry. The negative zero-field splitting parameter D assigned to all studied corroles is explained by the asymmetric π-electron withdrawal effect caused by perfluorinated peripheral aryl groups, which force the triplet electron spins to align in head-to-tail configuration. The effect of the axial ligands on the photoexcited triplet state properties of the corroles is correlated with their different efficiency to perform photoassisted aerobic oxygenation of some organic molecules. This is explained by the dependence of the main parameters of the photoexcited complexes on the interaction between the central ion and corrole π-system. This interaction is strongly influenced by axial ligands coordination, affecting the macrocycle symmetry, planarity, and rigidity.

Nanowires have the potential to play an important role for next-generation light-emitting diodes. In this work, we present a growth scheme for radial nanowire quantum-well structures in the AlGaInP material system using a GaInP nanowire core as a template for radial growth with GaInP as the active layer for emission and AlGaInP as charge carrier barriers. The different layers were analyzed by X-ray diffraction to ensure lattice-matched radial structures. Furthermore, we evaluated the material composition and heterojunction interface sharpness by scanning transmission electron microscopy energy dispersive X-ray spectroscopy. The electro-optical properties were investigated by injection luminescence measurements. The presented results can be a valuable track toward radial nanowire light-emitting diodes in the AlGaInP material system in the red/orange/yellow color spectrum.

The photoexcited triplet states of three 5,10, 15-tris(pentafluorophenyl)corroles (tpfc), hosting Sn(IV) and Al(III) in their core, namely, Sn(Cl)(tpfc), Al(pyr)2(tpfc) and Al(pyr)2(tpfc-Br8), were studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy in the nematic liquid crystal E7. Only two of these metallocorroles, namely, Sn(Cl)(tpfc) and Al(pyr)2(tpfc-Br8), exhibit TREPR spectra following pulsed laser excitation. This result is rationalized in terms of a very low quantum yield of triplet formation in Al(pyr)2(tpfc). Analysis of the spin polarized Q-band (34 GHz) EPR spectra of Sn(Cl)(tpfc) and Al(pyr)2(tpfc-Br8) provides detailed information on the magnetic and kinetic parameters of the triplet states as well as on the molecular ordering of the complexes in the liquid crystal. With the assignment of the zero-field splitting parameterD AX, AY, which is attributed to a large increase in the spin-orbit coupling strength arising from the peripheral bromine atoms on the corrole skeleton.

Light-driven multi-step intramolecular electron transfer in a rod-like triad, in which two of the three redox components are linked by three hydrogen bonds, was studied by time-resolved electron paramagnetic resonance (TREPR) and optical spectroscopies. One part of the molecule consists of a p-methoxyaniline primary electron donor (MeOAn) covalently linked to a 4-aminonaphthalene-1, 8-dicarboximide (6ANI) chromophoric electron acceptor (MeOAn-6ANI). The unsubstituted dicarboximide of 6ANI serves as one half of a hydrogen bonding receptor pair. The other half of the receptor pair consists of a melamine linked to a naphthalene-1,8:4,5-bis(dicarboximide) (NI) secondary electron acceptor (MEL-NI). TREPR spectroscopy is used to probe the electronic interaction between the radicals within the photogenerated, spin-correlated radical ion pair MeOAn +-6ANI/MEL-NI −. The results are compared to those obtained in earlier studies in which MeOAn-6ANI is covalently linked to NI through a 2,5-dimethylphenyl group (MeOAn-6ANI-Ph-NI). We show that the electronic coupling between the oxidized donor and reduced acceptor in the hydrogen-bonded radical ion pair MeOAn +-6ANI/MEL-NI − is very similar to that of MeOAn +-6ANI-Ph-NI −.

Time-resolved electron paramagnetic resonance (TREPR) spectroscopy was used to study two functionalized fullerenes consisting of a C60 moiety covalently linked to TEMPO radical via spacers of different length. Photoinduced electron spin polarization (ESP) reflecting a non-Boltzmann population within the energy levels of the spin system was observed in the electronic ground and excited states. Both fullerenes are characterized by a sign inversion of their TREPR spectra. A new mechanism of ESP generation was suggested to explain the experimental results. This mechanism, termed as the reversed quartet mechanism (RQM), includes the intersystem crossing process, which generates ESP in the excited trip-doublet and trip-quartet (2T1 and 4T1) states. This ISC is accompanied by ESP transfer to the ground state (2S0) by either electron-transfer reaction (in our case via charge transfer state, 2CT, i.e., 2T1 → 2CT → 2S0) or internal conversion, 2T1 → 2S0.

Light-induced spin-polarized transient EPR spectra are reported for several water-soluble copper porphyrins. The spectra are assigned to the doublet ground state, with emissive spin polarization resulting from photoexcitation and subsequent electronic relaxation. In contrast to other systems for which polarization of a doublet ground state has been observed, the exchange interactions in the copper porphyrins are strong and the geometry is fixed. It is proposed that intersystem crossing from the photoexcited trip-doublet to the trip-quartet state can lead to net polarization of the spin system and that this polarization is maintained during electronic decay, possibly via charge-transfer and exciplex states. The intensity of the observed spin polarization is essentially independent of the molecular orientation in the external field, but is strongly dependent on the nature of the charged peripheral groups. Possible reasons for this behavior are discussed.

Electron paramagnetic resonance (EPR) spectroscopy was used to study the interactions between stable free radicals and gold nanoparticles. The nitroxyl free radicals used were TEMPO, TEMPAMINE, and TEMPONE. Two sizes of Au particles, 15 and 2.5 nm in diameter, were synthesized to investigate the interactions with the metallic particles. We find that the EPR signal is reduced upon adsorption of the radicals onto the 15 nm Au particle surface. Despite the strong adsorption of TEMPAMINE on the particles, the signal intensity recovers upon the introduction of a high concentration of ethanolamine to the solution. The signal reduction was proportional to the concentration of Au particles, and the signal totally disappeared at high concentrations of Au particles. Possible explanations of the signal reduction are discussed in this Article. We propose that the reduction in signal intensity arises from exchange interactions between the unpaired electrons of the adsorbed radicals and conduction-band electrons of the metallic particles. In addition, in the presence of oxygen, the adsorbed TEMPAMINE radicals are catalytically oxidized to the carbonyl derivative, TEMPONE. A mechanism for this unexpected catalytic reaction is proposed.

Radial GaInP/AlGaInP nanowire array light-emitting diodes (LEDs) are promising candidates for novel high-efficiency solid state lighting due to their potentially large strain-free active emission volumes compared to planar LEDs. Moreover, by proper tuning of the diameter of the nanowires, the fraction of emitted light extracted can be significantly enhanced compared to that of planar LEDs. Reports so far on radial growth of nanowire LED structures, however, still point to significant challenges related to obtaining defect-free radial heterostructures. In this work, we present evidence of optically active growth-induced defects in a fairly broad energy range in vertically processed radial GaInP/AlGaInP quantum well nanowire array LEDs using a variety of complementary experimental techniques. In particular, we demonstrate strong infrared electroluminescence in a spectral range centred around 1 eV (1.2 μm) in addition to the expected red light emission from the quantum well. Spatially resolved cathodoluminescence studies reveal a patchy red light emission with clear spectral features along the NWs, most likely induced by variations in QW thickness, composition and barriers. Dark areas are attributed to infrared emission generated by competing defect-assisted radiative transitions, or to trapping mechanisms involving non-radiative recombination processes. Possible origins of the defects are discussed.

The unit of y axis in Fig. 2 in the original version of this article was unfortunately wrongly written on page 2856, instead of m−2. (Figure Presented.).

The photoexcited triplet state of the two new porphyrin isomers, octaethylhemiporphycene (OEHPC) and octaethylcorrphycene (OECPC), embedded in isotropic (toluene) and an anisotropic liquid crystal matrices was studied by time-resolved electron paramagnetic resonance (EPR) spectroscopy in the temperature range of 150–300 K. The magnetic, kinetic and orientation parameters were determined and interpreted in terms of structure, symmetry and dynamic. Analysis of the results suggests that at T ⩾250 K, the orientation and packing of the chromophores result from a discrete solid-like jumps mechanism, which is more efficient for OECPC. This difference is rationalized in terms of differences in the symmetry of the two chromophores.

We report on an unusual intramolecular electron transfer following the photoexcitation of the ground, charge separated, state of the zwitterionic compound, tetraphenylhexaazaanthracene, which results in a neutral biradical product. The reaction, studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy, could be carried out only when liquid crystals were used as the orienting solvents. On the basis of the analysis of the magnetic, kinetic, and polarization parameters of the TREPR spectra, the radical pair genesis is attributed to an intramolecular electron transfer from the singlet excited state.

We report on intramolecular electron and/or energy transfer (IET and IEnT), in two photoexcited electrostatically bound dimers: (1) zinc(II) tetrakis(4-sulfonatophenyl)porphyrin and magnesium(II) tetrakis(4-N,N,N-trimethylanilinium)porphyrin, [ZnTSPP−MgTTAP]; and (2) zinc(II) tetrakis(4-N,N,N-trimethylanilinium)porphyrin and magnesium(II) tetrakis(4-sulfonatophenyl)porphyrin, [ZnTTAP−MgTSPP]. The photochemical properties of both dimers, dissolved in dimethylsulfoxide−glycerol (DMSO−Glyc, 140 K), were investigated by continuous wave time-resolved electron paramagnetic resonance (CW-TREPR) spectroscopy. It is demonstrated that by changing the peripheral substitution of the particular metalloporphyrin in the dimer, an IET route, parallel to the singlet IEnT, can be triggered. Thus, singlet IEnT from the Zn to the Mg constituent is the only process in [ZnTSPP−MgTTAP], while IET to produce a charge-separated state, [ZnTTAP•+−MgTSPP•-] accompanies the IEnT in [ZnTTAP−MgTSPP].

Light-driven intramolecular electron transfer (ET) and energy transfer (EnT) processes in two rotaxanes, the first containing two free base porphyrins and C(60) fullerene moieties incorporated around a Cu(I)bisphenanthroline core ((H(2)P)(2)-Cu(I)(phen)(2)-C(60)) and a second lacking the fullerene moiety ((H(2)P)(2)-Cu(I)(phen)(2)), were studied by X-band (9.5 GHz) time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The experiments were performed in frozen toluene and ethanol and different phases of the nematic liquid crystal (E-7). It is demonstrated that the ET and EnT processes in the (H(2)P)(2)-Cu(I)(phen)(2)-C(60) rotaxane in different media result in the formation of the same charge-separated state, namely (H(2)P)(2)(•+)-Cu(I)(phen)(2)(•-)-C(60), while photoexcitation of the (H(2)P)(2)-Cu(I)(phen)(2) rotaxane does not induce noticeable transfer processes in these matrices. The results are discussed in terms of the high conformational mobility of the rotaxanes, which enables changes in the molecular topography and resultant modification of the rates and routes of photoinduced processes occurring in these systems. The parameters of the transfer processes are compared with those obtained in our previous study of (ZnP)(2)-Cu(I)(phen)(2)-C(60) and (ZnP)(2)-Cu(I)(phen)(2) rotaxanes under the same experimental conditions.

Photoexcited states of three Rh(III) 5,10,15-tris(pentafluorophenyl)corroles coordinated by different axial ligands; namely, triphenylphosphine P(C(6)H(5))(3) group (1), pyridine C(6)H(5)N group (2), and two pyridine groups (3) were studied by X- and Q-band time-resolved electron paramagnetic resonance (TREPR) in frozen toluene and liquid crystal E-7. Transient mutations were utilized to identify multiplicity of the detected paramagnetic species. The spectra of 1 and 2 were assigned to triplet ((3)pipi*) states, while contributions of triplet ((3)dd and charge transfer (3)CT) and quintet ((5)dd) states were revealed in the spectrum of 3. The results are interpreted in terms of a peculiar nature of transition metal complexes with the unfilled d-shell, where close lying electronic states of different multiplicities may be mixed through configurational, spin-orbit, and vibronic coupling. From the EPR spectra, the spin-orbit coupling constant was estimated to be about 25 cm(-1). It is shown that different axial ligation of complexes shifts the relative energy of the excited states and, consequently, leads to population of different states. Plausible explanations of the effects governing unusual spectral and dynamic parameters of the photoexcited Rh corrole complexes are presented.

The photoexcited triplet state of the thiouredopyrenetrisulfonate (TUPS) covalently linked to (1) α-acetyl lysine (3*TUPS-L) and (2) lysine residue 122 of azurin (3*TUPS-A) were studied by time-resolved EPR spectroscopy. The triplet spectra of 3*TUPS-L and 3*TUPS-A dissolved in water−glycerol (1:1) mixture, at freezing temperatures, show different temperature dependence. While in the case of 3*TUPS-L, no apparent dynamic effects were noticed, 3*TUPS-A exhibits a strong effect at 230 K. The dynamic effects were treated by line shape analysis with the discrete jumps model, suggesting that the environment in which 3*TUPS-A is embedded is not frozen, allowing for fast molecular motion with a correlation time of ∼1.8 × 10-10 s. These results are explained by the unique properties of the protein-bound water layers, which are kept in the liquid state even at relatively low temperatures, where the bulk water is frozen.

Three corroles, which differ by their cavity's core, namely, diamagnetic free-base tris(pentafluorophenyl)corrole and its gallium(III) complex and the paramagnetic oxo-chromium(V) complex, were studied by steady-state and time-resolved electron paramagnetic resonance (EPR) spectroscopy. The magnetic and orientational parameters of the corroles, oriented in a nematic liquid crystal, were determined and interpreted in terms of their structure, geometry, and excited states spin dynamics. It was shown that both diamagnetic corroles, photoexcited to their triplet states, exhibit similar EPR line shapes, which is characterized by a negative zero-field splitting parameter, D, whose origin is due to molecular "stretching". Photoexcited Cr(V)O-corrole exhibits polarized ground-state EPR spectrum in emission mode. This polarization stems from the sequence of photophysical and photochemical reactions, involving the formation of the trip-quartet/trip-doublet composite states and their selective quenching via a charge transfer state.

Light-driven electron transfer (ET) and energy transfer (EnT) in a self-assembled via axial coordination Zn-porphyrin-pyridylfullerene (ZnP-PyrF) complex were studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy at 9.5 GHz (X-band) and 95 GHz (W-band). The studies over a wide temperature range were carried out in media of different polarity, including isotropic toluene and tetrahydrofuran (THF), and anisotropic nematic liquid crystals (LCs), E-7 and ZLI-4389. At low temperatures (frozen matrices), photoexcitation of the ZnP donor results mainly in singlet-singlet EnT to the pyridine-appended fullerene acceptor. In fluid phases ET is the dominant process. Specifically, in isotropic solvents the generated radical pairs (RPs) are long-lived, with lifetimes exceeding that observed for covalently linked donor-acceptor systems. It is concluded that in liquid phases of both polar and nonpolar solvents the separation of the tightly bound complex into the more loosely bound structure slows down the back ET (BET) process. Photoexcitation of the donor in fluid phases of LCs does not result in the creation of the long-lived RPs, since the ordered LC matrix hinders the separation of the complex constituents. As a result, fast intramolecular BET takes place in the tightly bound complex. Contrarily to the behavior of covalently linked donor-acceptor systems in different LCs, the polarity of the LC matrix affects the ET process. Moreover, in contrast to covalently linked D-s-A systems, utilization of LCs for the coordinatively linked D-s-A complexes does not reduce the ET rates significantly.