Publications


Towards higher electron mobility in modulation doped GaAs/AlGaAs core shell nanowires

Nanoscale Royal Society of Chemistry 9 (2017) 7839-7846

JL Boland, G Tutuncuoglu, JQ Gong, S Conesa-Boj, CL Davis, LM Herz, A Fontcuberta i Morral, M Johnston

Precise control over the electrical conductivity of semiconductor nanowires is a crucial prerequisite for implementation into novel electronic and optoelectronic devices. Advances in our understanding of doping mechanisms in nanowires and their influence on electron mobility and radiative efficiency are urgently required. Here, we investigate the electronic properties of n-type modulation doped GaAs/AlGaAs nanowires via optical pump terahertz (THz) probe spectroscopy and photoluminescence spectroscopy over the temperature range 5K-300K. We directly determine an ionisation energy of 6.7±0.5meV (T = 52K) for the Si donors that create the modulation doping in the AlGaAs shell. We further elucidate the temperature dependence of the electron mobility, photoconductivity lifetime and radiative efficiency, and determine the charge-carrier scattering mechanisms that limit electron mobility. We show that below the donor ionization temperature, charge scattering is limited by interactions with interfaces, leading to an excellent electron mobility of 4360±380cm2V-1s-1 at 5 K. Above the ionization temperature, polar scattering via longitudinal optical (LO) phonons dominates, leading to a room temperature mobility of 2220±130cm2V-1s-1. In addition, we show that the Si donors effectively passivate interfacial trap states in the nanowires, leading to prolonged photoconductivity lifetimes with increasing temperature, accompanied by an enhanced radiative efficiency that exceeds 10% at room temperature.


The entangled triplet pair state in acene and heteroacene materials.

Nature Communications Springer Nature 8 (2017) 15953

CK Yong, AJ Musser, SL Bayliss, S Lukman, H Tamura, O Bubnova, RK Hallani, A Meneau, R Resel, M Maruyama, S Hotta, L Herz, D Beljonne, JE Anthony, J Clark, H Sirringhaus

Entanglement of states is one of the most surprising and counter-intuitive consequences of quantum mechanics, with potent applications in cryptography and computing. In organic materials, one particularly significant manifestation is the spin-entangled triplet-pair state, which mediates the spin-conserving fission of one spin-0 singlet exciton into two spin-1 triplet excitons. Despite long theoretical and experimental exploration, the nature of the triplet-pair state and inter-triplet interactions have proved elusive. Here we use a range of organic semiconductors that undergo singlet exciton fission to reveal the photophysical properties of entangled triplet-pair states. We find that the triplet pair is bound with respect to free triplets with an energy that is largely material independent (∼30 meV). During its lifetime, the component triplets behave cooperatively as a singlet and emit light through a Herzberg-Teller-type mechanism, resulting in vibronically structured photoluminescence. In photovoltaic blends, charge transfer can occur from the bound triplet pairs with >100% photon-to-charge conversion efficiency.


Polarisation-controlled single photon emission at high temperatures from InGaN quantum dots

Nanoscale Royal Society of Chemistry 9 (2017) 9421-9427

T Wang, TJ Puchtler, T Zhu, JC Jarman, LP Nuttall, RA Oliver, RA Taylor

Solid-state single photon sources with polarisation control operating beyond the Peltier cooling barrier of 200 K are desirable for a variety of applications in quantum technology. Using a non-polar InGaN system, we report the successful realisation of single photon emission with a g<sup>(2)</sup>(0) of 0.21, a high polarisation degree of 0.80, a fixed polarisation axis determined by the underlying crystallography, and a GHz repetition rate with a radiative lifetime of 357 ps at 220 K in semiconductor quantum dots. The temperature insensitivity of these properties, together with the simple planar epitaxial growth method and absence of complex device geometries, demonstrates that fast single photon emission with polarisation control can be achieved in solid-state quantum dots above the Peltier temperature threshold, making this system a potential candidate for future on-chip applications in integrated systems.


High-temperature performance of non-polar (11–20) InGaN quantum dots grown by a quasi-two-temperature method

Physica Status Solidi B: Basic Solid State Physics Wiley 254 (2017) 1600724

T Wang, TJ Puchtler, T Zhu, JC Jarman, RA Oliver, RA Taylor

Non-polar (11–20) a-plane InGaN quantum dots (QDs) are one of the strongest candidates to achieve on-chip applications of polarised single photon sources, which require a minimum operation temperature of ∼200 K under thermoelectrically cooled conditions. In order to further improve the material quality and optical properties of a-plane InGaN QDs, a quasi-two-temperature (Q2T) method has been developed, producing much smoother underlying InGaN quantum well than the previous modified droplet epitaxy (MDE) method. In this work, we compare the emission features of QDs grown by these two methods at temperatures up to 200 K. Both fabrications methods are shown to be able to produce QDs emitting around the thermoelectric cooling barrier. The sample fabricated by the new Q2T method demonstrates more stable operation, with an order of magnitude higher intensity at 200 K comparing to the comparable QDs grown by MDE. A detailed discussion of the possible mechanisms that result in this advantage of slower thermal quenching is presented. The use of this alternative fabrication method hence promises more reliable operation at temperatures even higher than the thermoelectric cooling threshold, and facilitates the on-going development of high temperature polarised single photon sources based on a-plane InGaN QDs.


Impact of the Halide Cage on the Electronic Properties of Fully Inorganic Cesium Lead Halide Perovskites

ACS ENERGY LETTERS (2017)

Z Yang, A Surrente, K Galkowski, A Miyata, O Portugall, RJ Sutton, AA Haghighirad, HJ Snaith, DK Maude, P Plochocka, R Nicholas


Crystallization kinetics and morphology control of formamidinium-cesium mixed-cation lead mixed-halide perovskite via tunability of the colloidal precursor solution

Advanced Materials Wiley 29 (2017) 1-8

D McMeekin, Z Wang, W Rehman, F Pulvirenti, JB Patel, NK Noel, MB Johnston, Marder, L Herz, HJ Snaith

The meteoric rise of the field of perovskite solar cells has been fueled by the ease with which a wide range of high-quality materials can be fabricated via simple solution processing methods. However, to date, little effort has been devoted to understanding the precursor solutions, and the role of additives such as hydrohalic acids upon film crystallization and final optoelectronic quality. Here, a direct link between the colloids concentration present in the [HC(NH2 )2 ]0.83 Cs0.17 Pb(Br0.2 I0.8 )3 precursor solution and the nucleation and growth stages of the thin film formation is established. Using dynamic light scattering analysis, the dissolution of colloids over a time span triggered by the addition of hydrohalic acids is monitored. These colloids appear to provide nucleation sites for the perovskite crystallization, which critically impacts morphology, crystal quality, and optoelectronic properties. Via 2D X-ray diffraction, highly ordered and textured crystals for films prepared from solutions with lower colloidal concentrations are observed. This increase in material quality allows for a reduction in microstrain along with a twofold increase in charge-carrier mobilities leading to values exceeding 20 cm(2) V(-1) s(-1) . Using a solution with an optimized colloidal concentration, devices that reach current-voltage measured power conversion efficiency of 18.8% and stabilized efficiency of 17.9% are fabricated.


V-Shaped Hole-Transporting TPD Dimers Containing Troger's Base Core

JOURNAL OF PHYSICAL CHEMISTRY C 121 (2017) 10267-10274

T Braukyla, N Sakai, M Daskeviciene, V Jankauskas, E Kamarauskas, R Komskis, T Malinauskas, S Jursenas, HJ Snaith, V Getautis

V-shaped hole transporting materials based on N,N,N′,N′-tetraarylbenzidine (TPD)-type moieties conjoined by Tröger’s base core were synthesized and investigated. These hole transporting materials were obtained by a three-step synthetic method, are fully amorphous, and demonstrate high glass transition temperatures and good thermal and morphological stability. Relatively high charge mobility (up to 0.036 cm2 V –1 s–1) was measured in these hole transporting materials, exceeding that of corresponding methyl and methoxy substituted TPD analogues without TB core by more than 2 orders of magnitude. Determined ionization potential and charge mobility values permit use of the synthesized compounds as hole transporting materials in fabrication of perovskite solar cells.


Efficient ambient-air-stable solar cells with 2D–3D heterostructured butylammonium-caesium-formamidinium lead halide perovskites

Nature Energy Springer Nature 6 (2017) 17135-

Z Wang, Q Lin, FP Chmiel, N Sakai, L Herz, HJ Snaith

<p>Perovskite solar cells are remarkably efficient; however, they are prone to degradation in water, oxygen and ultraviolet light. Cation engineering in 3D perovskite absorbers has led to reduced degradation. Alternatively, 2D Ruddlesden–Popper layered perovskites exhibit improved stability, but have not delivered efficient solar cells so far. Here, we introduce <em>n</em>-butylammonium cations into a mixed-cation lead mixed-halide FA<sub>0.83</sub>Cs<sub>0.17</sub>Pb(I<sub><em>y</em></sub>Br<sub>1−<em>y</em></sub>)<sub>3</sub> 3D perovskite. We observe the formation of 2D perovskite platelets, interspersed between highly orientated 3D perovskite grains, which suppress non-radiative charge recombination. We investigate the relationship between thin-film composition, crystal alignment and device performance. Solar cells with an optimal butylammonium content exhibit average stabilized power conversion efficiency of 17.5 ± 1.3% with a 1.61-eV-bandgap perovskite and 15.8 ± 0.8% with a 1.72-eV-bandgap perovskite. The stability under simulated sunlight is also enhanced. Cells sustain 80% of their ‘post burn-in’ efficiency after 1,000 h in air, and close to 4,000 h when encapsulated.</p>


Band-tail recombination in hybrid lead iodide perovskite

Advanced Functional Materials Wiley (2017)

AD Wright, RL Milot, GE Eperon, HJ Snaith, L Johnston, MB Herz

Traps limit the photovoltaic efficiency and affect the charge transport of optoelectronic devices based on hybrid lead halide perovskites. Understanding the nature and energy scale of these trap states is therefore crucial for the development and optimization of solar cell and laser technology based on these materials. Here, the low-temperature photoluminescence of formamidinium lead triiodide (HC(NH2)2PbI3) is investigated. A power-law time dependence in the emission intensity and an additional low-energy emission peak that exhibits an anomalous relative Stokes shift are observed. Using a rate-equation model and a Monte Carlo simulation, it is revealed that both phenomena arise from an exponential trap-density tail with characteristic energy scale of ≈3 meV. Charge-carrier recombination from sites deep within the tail is found to cause emission with energy downshifted by up to several tens of meV. Hence, such phenomena may in part be responsible for open-circuit voltage losses commonly observed in these materials. In this high-quality hybrid perovskite, trap states thus predominantly comprise a continuum of energetic levels (associated with disorder) rather than discrete trap energy levels (associated, e.g., with elemental vacancies). Hybrid perovskites may therefore be viewed as classic semiconductors whose bandstructure picture is moderated by a modest degree of energetic disorder.


Temperature-dependent fine structure splitting in InGaN quantum dots

Applied Physics Letters AIP Publishing 111 (2017) 053101

T Wang, TJ Puchtler, T Zhu, JC Jarman, CC Kocher, RA Oliver, R Taylor

We report the experimental observation of temperature-dependent fine structure splitting in semiconductor quantum dots using a non-polar (11-20) a-plane InGaN system, up to the on-chip Peltier cooling threshold of 200 K. At 5 K, a statistical average splitting of 443 ± 132 μeV has been found based on 81 quantum dots. The degree of fine structure splitting stays relatively constant for temperatures less than 100 K and only increases above that temperature. At 200 K, we find that the fine structure splitting ranges between 2 and 12 meV, which is an order of magnitude higher than that at low temperatures. Our investigations also show that phonon interactions at high temperatures might have a correlation with the degree of exchange interactions. The large fine structure splitting at 200 K makes it easier to isolate the individual components of the polarized emission spectrally, increasing the effective degree of polarization for potential on-chip applications of polarized single-photon sources.


Impact of the halide cage on the electronic properties of fully inorganic cesium lead halide perovskites

ACS Energy Letters American Chemical Society 2 (2017) 1621–1627-

Z Yang, A Surrente, K Galkowski, A Miyata, O Portugall, R Sutton, AA Haghighirad, HJ Snaith, DK Maude, P Plochocka, RJ Nicholas

Perovskite solar cells with record power conversion efficiency are fabricated by alloying both hybrid and fully inorganic compounds. While the basic electronic properties of the hybrid perovskites are now well understood, key electronic parameters for solar cell performance, such as the exciton binding energy of fully inorganic perovskites, are still unknown. By performing magneto-transmission measurements, we determine with high accuracy the exciton binding energy and reduced mass of fully inorganic CsPbX3 perovskites (X = I, Br, and an alloy of these). The well-behaved (continuous) evolution of the band gap with temperature in the range of 4–270 K suggests that fully inorganic perovskites do not undergo structural phase transitions like their hybrid counterparts. The experimentally determined dielectric constants indicate that at low temperature, when the motion of the organic cation is frozen, the dielectric screening mechanism is essentially the same for both hybrid and inorganic perovskites and is dominated by the relative motion of atoms within the lead halide cage.


A nanophotonic structure containing living photosynthetic bacteria

Small Wiley‐VCH Verlag 13 (2017) 1701777

D Coles, LC Flatten, T Sydney, E Hounslow, SK Saikin, A Aspuru-Guzik, V Vedral, JK Tang, RA Taylor, JM Smith, DG Lidzey

Photosynthetic organisms rely on a series of self‐assembled nanostructures with tuned electronic energy levels in order to transport energy from where it is collected by photon absorption, to reaction centers where the energy is used to drive chemical reactions. In the photosynthetic bacteria Chlorobaculum tepidum, a member of the green sulfur bacteria family, light is absorbed by large antenna complexes called chlorosomes to create an exciton. The exciton is transferred to a protein baseplate attached to the chlorosome, before migrating through the Fenna–Matthews–Olson complex to the reaction center. Here, it is shown that by placing living Chlorobaculum tepidum bacteria within a photonic microcavity, the strong exciton–photon coupling regime between a confined cavity mode and exciton states of the chlorosome can be accessed, whereby a coherent exchange of energy between the bacteria and cavity mode results in the formation of polariton states. The polaritons have energy distinct from that of the exciton which can be tuned by modifying the energy of the optical modes of the microcavity. It is believed that this is the first demonstration of the modification of energy levels within living biological systems using a photonic structure.


Direct generation of linearly polarized single photons with a deterministic axis in quantum dots

Nanophotonics De Gruyter Open 6 (2017) 1175–1183-

T Wang, TJ Puchtler, SK Patra, T Zhu, M Ali, T Badcock, T Ding, RA Oliver, S Schulz, RA Taylor

We report the direct generation of linearly polarized single photons with a deterministic polarization axis in self-assembled quantum dots (QDs), achieved by the use of non-polar InGaN without complex device geometry engineering. Here we present a comprehensive investigation of the polarization properties of these QDs and their origin with statistically significant experimental data and rigorous k·p modelling. The experimental study of 180 individual QDs allow us to compute an average polarization degree of 0.90, with a standard deviation of only 0.08. When coupled with theoretical insights, we show that these QDs are highly insensitive to size differences, shape anisotropies, and material content variations. Furthermore, 91% of the studied QDs exhibit a polarization axis along the crystal [1-100] axis, with the other 9% polarized orthogonal to this direction. These features give non-polar InGaN QDs unique advantages in polarization control over other materials, such as conventional polar nitride, InAs, or CdSe QDs. Hence, the ability to generate single photons with polarization control makes non-polar InGaN QDs highly attractive for quantum cryptography protocols.


How to avoid artifacts in surface photovoltage measurements: a case study with halide perovskites

Journal of Physical Chemistry Letters American Chemical Society 8 (2017) 2941–2943-

G Hodes, I Levine, HJ Snaith, P Nayak


Near-infrared and short-wavelength infrared photodiodes based on dye-perovskite composites

Advanced Functional Materials Wiley 27 (2017) 1702485

Q Lin, Z Wang, M Young, JB Patel, RL Milot, L Martinez Maestro, RR Lunt, HJ Snaith, MB Johnston, L Herz

Organohalide perovskites have emerged as promising light-sensing materials because of their superior optoelectronic properties and low-cost processing methods. Recently, perovskite-based photodetectors have successfully been demonstrated as both broadband and narrowband varieties. However, the photodetection bandwidth in perovskite-based photodetectors has so far been limited to the near-infrared regime owing to the relatively wide band gap of hybrid organohalide perovskites. In particular, short-wavelength infrared photodiodes operating beyond 1 μm have not yet been realized with organohalide perovskites. In this study, narrow band gap organic dyes are combined with hybrid perovskites to form composite films as active photoresponsive layers. Tuning the dye loading allows for optimization of the spectral response characteristics and excellent charge-carrier mobilities near 11 cm 2 V -1 s -1 , suggesting that these composites combine the light-absorbing properties or IR dyes with the outstanding charge-extraction characteristics of the perovskite. This study demonstrates the first perovskite photodiodes with deep near-infrared and short-wavelength infrared response that extends as far as 1.6 μm. All devices are solution-processed and exhibit relatively high responsivity, low dark current, and fast response at room temperature, making this approach highly attractive for next-generation light-detection techniques.


Processing Solvent-Dependent Electronic and Structural Properties of Cesium Lead Triiodide Thin Films.

The journal of Physical Chemistry Letters (2017) 4172-4176

AJ Ramadan, LA Rochford, S Fearn, HJ Snaith

Cesium lead triiodide (CsPbI3) is an attractive material for photovoltaic applications due to its appropriate band gap, strong optical absorption, and high thermal stability. However, the perovskite phase suffers from moisture induced structural instability. Previous studies have utilized a range of solvent systems to establish the role of solvent choice in structural instabilities. Despite this, effects of different solvents on the electronic structure of this material have not been compared. We report substantial chemical and compositional differences in thin films of CsPbI3 prepared from a range of solvent systems. We confirm via X-ray diffraction thin films formed from DMF, DMSO, and a mixture of these solvent systems share the same crystal structure. However, secondary ion mass spectrometry, X-ray photoelectron spectroscopy, and low energy ion scattering measurements reveal significant differences between films processed via different solvent systems. Our findings reveal the critical impact solvents have upon compositional stoichiometry and thin-film morphology.


Tuning Biocompatible Block Copolymer Micelles by Varying Solvent Composition: Core/Corona Structure and Solvent Uptake

Macromolecules 50 (2017) 4322-4334

TJ Cooksey, A Singh, KM Le, S Wang, EG Kelley, L He, S Vajjala Kesava, ED Gomez, BE Kidd, LA Madsen, ML Robertson

© 2017 American Chemical Society. Block copolymer micelles enable the formation of widely tunable self-assembled structures in liquid phases, with applications ranging from drug delivery to personal care products to nanoreactors. In order to understand fundamental aspects of micelle assembly and dynamics, the structural properties and solvent uptake of biocompatible poly(ethylene oxide-b-ϵ-caprolactone) (PEO-PCL) diblock copolymers in deuterated water (D 2 O)/tetrahydrofuran (THF-d 8 ) mixtures were investigated with a combination of small-angle neutron scattering, nuclear magnetic resonance, and transmission electron microscopy. PEO-PCL block copolymers, of varying molecular weight yet constant block ratio, formed spherical micelles through a wide range of solvent compositions. Varying the solvent composition from 10 to 60 vol % THF-d 8 in D 2 O/THF-d 8 mixtures was a convenient means of varying the core-corona interfacial tension in the micelle system. An increase in THF-d 8 content in the bulk solvent increased the solvent uptake within the micelle core, which was comparable for the two series, irrespective of the polymer molecular weight. Whereas the smaller molecular weight micelle series exhibited a decrease in aggregation number with increasing THF-d 8 content in the bulk solvent, as anticipated due to changes in the core-corona interfacial tension, the aggregation number of the larger molecular weight series was surprisingly invariant with bulk solvent composition. Differences in the dependencies of the micelle size parameters (core radius and overall micelle radius) on the solvent composition originated from the differing trends in aggregation number for the two micelle series. Incorporation of the known unimer content determined from NMR (described in the companion paper), and directly accounting for impacts of solvent swelling of the micelle core on the neutron scattering length density of the core, allowed refinement of and increased confidence in extracted micelle parameters. In summary, the two micelle series showed similar solvent uptake that was independent of the polymer molecular weight yet significantly different dependencies of their aggregation number and size parameters on the solvent composition.


Broadband single-nanowire photoconductive Terahertz detectors

CLEO: Science and Innovations Optical Society of America (2017)
Part of a series from Conference on Lasers and Electro-Optics

K Peng, P Parkinson, Q Gao, J Boland, Z Li, F Wang, YC Wenas, CL Davies, L Fu, M Johnston, HH Tan, C Jagadish, I Ieee

Broadband photoconductive terahertz detectors based on undoped InP single nanowires were demonstrated. By further design and growth of an axial n+-i-n+ structure to reduce the contact resistance, highly-sensitive n+-i-n+ InP single-nanowire terahertz detectors were achieved.


Optical polarization in mono and bilayer MoS 2

Current Applied Physics Elsevier 17 (2017) 1153-1157

Y Park, N Li, CCS Chan, BPL Reid, RA Taylor, H Im

Optical anisotropy in monolayer- and bilayer-MoS 2 was investigated by polarization resolved photoluminescence measurements. The photoluminescence of monolayer-MoS 2 is found to be partially polarized at 4.2 K and maintains this polarization characteristic up to room temperature, while the photoluminescence of bilayer-MoS 2 shows no obvious polarization. This polarization anisotropy is due to strain effects at the interface between the MoS 2 layer and the SiO 2 substrate, causing symmetry breaking of the MoS 2 charge distribution. Calculations using density functional theory of the electron density distribution of the monolayer- and bilayer-MoS 2 in the in-plane direction are also presented, giving support to our qualitative analysis.


Theoretical and experimental analysis of radiative recombination lifetimes in nonpolar InGaN/GaN quantum dots

PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS 254 (2017) ARTN 1600675

SK Patra, T Wang, TJ Puchtler, T Zhu, RA Oliver, RA Taylor, S Schulz

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