Energy-dependent Charge Research Articles

Overcoming Thermoelectric Trade-Offs through Lanthanide Oxide/Carbon Nanotube Hybrid Material

Owing to their abundance, cost-effectiveness, high Seebeck coefficient, and exceptional stability, oxide thermoelectric (TE) materials are becoming viable alternatives to traditional inorganic alloys.1-2 Yet, their direct integration into flexible TE devices is severely challenged by intrinsic brittleness and their mismatch with the ideal TE parameters—achieving a high Seebeck coefficient and low thermal conductivity without compromising electrical conductivity—at mid-to-low temperatures.3-4 Herein, we present a novel hybrid material that combines lanthanum molybdate (La2Mo2O9) with single-walled carbon nanotubes (SWCNTs) to overcome the TE trade-off relationship as well as constraints related to the form factor5 and suitability for use at room temperature.6-7 This study unveils a significant enhancement in electrical conductivity through the strategic facilitation of semiconductor-like charge carrier transport. This achievement is realized via the percolation of nanostructured La2Mo2O9 within single-walled carbon nanotube (SWCNT), thereby augmenting charge carrier mobility. The synergistic integration of SWCNTs into the La2Mo2O9 matrix judiciously narrows the bandgap of La2Mo2O9, leading to an elevated charge carrier concentration. Moreover, we harness an energy filtering effect, arising from the reduced valence band energy difference between La2Mo2O9 and SWCNT interfaces, to raise the Seebeck coefficient through energy-dependent charge carrier scattering. Crucially, the integration of ultralow thermal conductive La2Mo2O9 with SWCNTs induced interfacial phonon scattering, markedly reducing lattice thermal conductivity. This strategic modulation of these TE parameters led to enhanced ZT values at mid-to-room temperatures, overcoming hurdles traditionally faced by single-component oxide thermoelectrics. We could be able to extend our investigation to the development of a stable, flexible prototype-TE device, suggesting potential for widespread application in durable TE devices that function in the moderate temperature range. References Li et al. Energy Environ. Sci. 2012, 5, 7188-7195 Liu et al. J. Am. Chem. Soc. 2011, 133, 20112–20115Snyder et al. Nat. Mater. 2008, 7, 105–114He et al. Science 2017, 357, eaak9997Luo et al. ACS Appl. Mater. Interfaces 2022, 14, 36258–36267Chen et al. ACS Energy Lett. 2017, 2, 915–921Zhao et al. Energy Environ. Sci. 2014, 7, 2900-2924 Figure 1

Cu2+ at the surface / sub-surface region of CuFeO2 rhombohedral nanostructures facilitates specific capacitance (∼611 F g−1): An understanding of the solvation energy dependent charge transfer mechanism

In this work, the role Cu2+, formed at the surface or sub-surface region during hydrothermal reaction, on the charge storage mechanism of CuFeO2 rhombohedral electrode has been investigated. Our samples having chemical formula Cu0.102+Cu0.90+FeO2 and Cu0.152+Cu0.85+FeO2 synthesized in the presence of 6.0 and 8.0 g of NaOH, exhibit charge storage capacity ∼312 and 611 F g−1 at scan rate 2 mVs−1 with Na2SO4, an environment friendly electrolyte. Charge transfer mechanism across CuFeO2 electrode – electrolyte interface is found to be governed by Marcus's mechanism where solvation energy plays a predominant role.

Single-molecule photoelectron tunnelling spectroscopy.

Experimental mapping of transmission is essential for understanding and controlling charge transport through molecular devices and materials. Here we developed a single-molecule photoelectron tunnelling spectroscopy approach for mapping transmission beyond the HOMO-LUMO gap of the single diketopyrrolopyrrole molecule junction using an ultrafast-laser combined scanning tunnelling microscope-based break junction set-up at room temperature. Two resonant transport channels of ultrafast photocurrent are found by our photoelectron tunnelling spectroscopy, ranging from 1.31 eV to 1.77 eV, consistent with the LUMO + 1 and LUMO + 2 in the transmission spectrum obtained by density functional theory calculations. Moreover, we observed the modulation of resonant peaks by varying bias voltages, which demonstrates the ability to quantitatively characterize the effect of the electric field on frontier molecular orbitals. Our single-molecule photoelectron tunnelling spectroscopy offers an avenue that allows us to explore the nature of energy-dependent charge transport through single-molecule junctions.

Energy-Dependent Charge Separation in Conjugated Polymer Electrolyte Complexes

Supramolecular complexes have great potential as light-harvesting materials, as intermolecular structural organization can be manipulated through steric or electrostatic interactions to impact electronic coupling between energy and charge donors and acceptors. Here, we examine the relative rates and efficiencies of charge transfer in conjugated polymer electrolyte complexes (CPECs) based on a polythiophene electron donor (PTAK) and polyfluorene electron acceptor (PFPI). These CPECs are characterized by ordered polymer microstructures, as evident from spectral signatures of strong excitonic coupling within the PTAK component from steady-state UV–vis absorption spectra. We find that PTAK polarons are generated within tens of picoseconds through a combination of prompt and delayed charge separation following direct photoexcitation or energy transfer from PFPI. Further, we find that decreasing the length of charged PTAK side chains or increasing excitation energy increases the driving force for electron transfer to increase charge separation rates and yields for polarons, with the greatest relative yields observed at excitation energies that initiate PFPI-to-PTAK energy transfer. Charge separation between components can be rationalized from a canonical Marcus picture, whereby excess vibrational energy effectively lowers the barrier for PTAK-to-PFPI charge separation. This contrasts with the recently reported ultrafast (<100 fs) charge separation in small-molecule/polythiophene electrolyte complexes that is attributed to strong orbital mixing that gives rise to charge generation via CT exciton states. These results provide insights into conditions for realizing charge separation in concert with energy transfer in CPECs as light-harvesting materials.

Ultrafast glimpses of the excitation energy-dependent exciton dynamics and charge carrier mobility in Cs2SnI6 nanocrystals.

Advancements in photovoltaic research suggest that tin-based perovskites are potential alternatives to traditional lead-based structures. Cs2SnI6, specifically, stands out as a notable candidate, exhibiting impressive performance. However, its complete potential remains untapped primarily owing to the limited understanding of its photophysics. In light of this, this study aims to bridge this knowledge gap. To commence our study, we first executed theoretical investigations to locate the energetically diverse excitons within the Brillouin zone. Building on this knowledge, we then utilized transient absorption spectroscopy to investigate their temporal evolution. Herein, we observed the formation of high-energy excitons even when the incident photon energy was below the necessary threshold, which is quite distinctive and intriguing. Of particular interest is the generation of ultraviolet (UV) domain exciton using visible photons, which implies that Cs2SnI6 has the potential for efficient solar light harvesting. Tracking the kinetics revealed that this unique finding arises due to the intertwined formation and decay pathways undertaken by the different excitons, aided by intervalley scattering and phonon absorption processes. In addition, we found that the decay of the UV exciton was unusually slow. Transient mobility investigations were undertaken to probe the carrier transport behavior that further established hot carriers (HCs) in Cs2SnI6 to be highly mobile and susceptible to polaron formation. Overall, our findings demonstrate that Cs2SnI6 is a strong candidate for HC-based photovoltaics because it possesses all the prerequisites desired for such applications.

Large thermoelectric power factors by opening the band gap in semimetallic Heusler alloys

Efficient thermoelectric devices are typically built from semiconducting materials since their band gap leads to an asymmetry in the continuum of energy-dependent charge transport necessary for a large Seebeck effect. Due to their low cost and excellent mechanical properties half- and full-Heusler compounds have emerged as promising candidates for various applications. Here, we demonstrate how the thermopower of semimetallic Fe2VAl-based Heusler alloys can be significantly increased by transferring electronic states from within the pseudogap to higher energies. Density functional theory calculations predict that partial Ti and Si co-substitution in Fe2V1−xTixAl1−ySiy drives an opening of the pseudogap, that can be likewise achieved in Fe2V1−xTaxAl1−ySiy. Consequently, our experimental measurements on these co-substituted systems reveal exceptionally large thermoelectric power factors (7.3 − 10.3 mWm−1K−2 near room temperature), as well as average power factors up to 4.6 mWm−1K−2 in the most pratical temperature range (293 − 573 K). With this work we set the course for a general and reliable way of boosting the thermoelectric performance of semimetallic Heusler alloys.

Grain/grain‐boundary mediated dispersive photo‐current characteristics in close space sublimated micro‐granular CdTe films

Photo-current characteristics in close space sublimation processed multi-facetted Cadmium Telluride (CdTe) micro-granular films were studied under AM1.5, blue and green laser illuminations to assess the energy dependent charge transport characteristics. Photo-current under AM1.5 illumination (100 mW/cm 2 ) resulted in three times more than dark current while it was observed 2.4 and 2.5 times more in case of blue and green laser illuminations, respectively. While all the three illuminations (AM1.5, blue and green lasers) were capable to generate photo-electrons in CdTe, the variation in magnitude was articulated to the effect of grain boundaries present across the charge transport pathways as evidenced from the surface morphology examined by scanning electron microscopic studies which asserted that CdTe films were micro-granular in nature separated by grain boundaries. It is elucidated in the measurements that charge carriers under the illumination of AM1.5 light, acquired energy to overcome the grain boundaries easily compared to the charge carriers produced by blue and green light sources as evidenced from the photo-current measurements.

Enabling Efficient Charge Separation for Optoelectronic Conversion via an Energy-Dependent Z-Scheme n-Semiconductor-Metal-p-Semiconductor Schottky Heterojunction.

Achieving good charge separation while maintaining energetic electronic states in heterostructures is a challenge in designing efficient photocatalyst materials. Using first-principles calculations, we propose a Z-scheme Sn-m-Sp (n-semiconductor-metal-p-semiconductor) heterojunction as a viable avenue for achieving broad-spectrum sunlight absorption and, importantly, energy-dependent charge separation. As a proof-of-concept investigation, we investigated two ternary heterostructures, CdS-Au-PdO and SnO2-W-Ag2O, in which the electronic Fermi levels line up by virtue of the presence of an intermediate metal layer. A cascade of work functions in the relative order Wn < Wm < Wp drives electrons flowing from Sn to m and from m to Sp. The inner electric fields established at the Sn-m and m-Sp Schottky junctions selectively guide low-energy photoexcited electrons from Sn (CdS/SnO2) and low-energy holes from Sp (PdO/Ag2O) to the interposing Au or W metal, respectively. Importantly, relatively low Schottky barriers enforce charge separation by constraining high-energy photogenerated charges to the individual semiconductor layers. Operating together, these two mechanisms enable the achievement of highly efficient optoelectronic conversion.

AlInP X-ray photodiodes without incomplete charge collection noise

Previously, an Al0.52In0.48P p+-i-n+ spectroscopic photon counting X-ray photodiode with 2μm thick i layer (200μm diameter) was shown to suffer from energy-dependent incomplete charge collection noise (Lioliou et al., 2019). Subsequent measurements on a larger (400μm diameter) Al0.52In0.48P p+-i-n+ photodiode (reported here) revealed the presence of even greater incomplete charge collection noise. Given these findings, an expectation would have been that thicker Al0.52In0.48P structures (which would be required for efficient absorption of all but the softest X-rays) would have a greater incomplete charge collection noise contribution, thus suggesting that thick Al0.52In0.48P photodiodes may be of limited practicality as high performance detectors for photon counting X-ray spectroscopy. However, two new Al0.52In0.48P p+-i-n+ photodiodes (with 6μm i layers) were fabricated from material grown by the same technique (metalorganic vapour phase epitaxy) in the same reactor, and are now shown here to exhibit no signs of detectable incomplete charge collection noise under the illumination of X-ray photons of energy 4.95 keV to 21.17 keV. As such, now that greater experience has been built with Al0.52In0.48P, concerns about incomplete charge collection noise in X-ray detectors made from the material appear to have been unwarranted; the path towards thick Al0.52In0.48P X-ray detectors is now clear.

X-ray spectroscopy with an AlInP photodiode

Abstract A custom-made Al 0.52 In 0.48 P p + -i-n + circular mesa X-ray photodiode ( 200 μ m diameter; 2 μ m i layer thickness) has been investigated for its use in photon counting X-ray spectroscopy within the energy range 4.95 keV to 21.17 keV. This is the widest energy range so far used to characterize an AlInP X-ray detector. High-purity X-ray fluorescence calibration samples were excited by a Mo target X-ray tube to give characteristic energy photons with which to illuminate the detector. X-ray fluorescence spectra were accumulated with the Al 0.52 In 0.48 P X-ray detector coupled to a custom-made charge-sensitive preamplifier (both operated at a temperature of 30 °C ± 2 °C). The charge output linearly increased with X-ray photon energy. The energy resolution (Full Width at Half Maximum, FWHM) degraded from 0.99 keV ± 0.02 keV (80 e − rms) at 4.95 keV to 1.12 keV ± 0.02 keV (89 e − rms) at 21.17 keV. The increase of the FWHM with increased energy exceeded the increase expected as a consequence of Fano noise. Hence, the presence of energy dependent incomplete charge collection noise was suggested. The count rate linearly increased with increasing incident X-ray flux across the investigated flux range, up to ∼ 3 . 8 × 105 photons s−1 cm−2.

Towards a first measurement of the free neutron bound beta decay detecting hydrogen atoms at a throughgoing beamtube in a high flux reactor

In addition to the common 3-body decay of the neutron n → pe-ν̅e there should exist an effective 2-body subset with the electron and proton forming a Hydrogen bound state with well defined total momentum, total spin and magnetic quantum numbers. The atomic spectroscopic analysis of this bound system can reveal details about the underlying weak interaction as it mirrors the helicity distributions of all outgoing particles. Thus, it is unique in the information it carries, and an experiment unravelling this information is an analogue to the Goldhaber experiment performed more than 60 years ago. The proposed experiment will search for monoenergetic metastable BoB H atoms with 326 eV kinetic energy, which are generated at the center of a throughgoing beamtube of a high-flux reactor (e.g., at the PIK reactor, Gatchina). Although full spectroscopic information is needed to possibly reveal new physics our first aim is to prove the occurrence of this decay and learn about backgrounds. Key to the detection is the identification of a monoerergtic line of hydrogen atoms occurring at a rate of about 1 s−1 in the environment of many hydrogen atoms, however having a thermal distribution of about room temperature. Two scenarios for velocity (energy) filtering are discussed in this paper. The first builds on an purely electric chopper system, in which metastable hydrogen atoms are quenched to their ground state and thus remain mostly undetectable. This chopper system employs fast switchable Bradbury Nielsen gates. The second method exploits a strongly energy dependent charge exchange process of metastable hydrogen picking up an electron while traversing an argon filled gas cell, turning it into manipulable charged hydrogen. The final detection of hydrogen occurs through multichannel plate (MCP) detector. The paper describes the various methods and gives an outlook on rates and feasibility at the PIK reactor in Gatchina.

Ion energy distribution in extremely high electric field discharges

Singly charged ion energy distribution functions (IEDFs) have been investigated by the particle-in-cell/Monte Carlo method in practical discharges at extremely high reduced electric field E/n, i.e. the ratio of electric field and gas density, up to 700 kTd in helium. It is found in high E/n regime that IEDFs deviate from Maxwellian form due to complex processes: (a) ions undergo a transition to runaway regime, (b) the copious energetic fast neutrals generated by charge exchange produce slow ions via ionization collisions, which load in the low-energy part of IEDF, as well as commonly assumed energy-dependent charge exchange cross section. For E/n > 100 kTd, the ratio of the volumetric ionization rate and charge exchange rate exceeds 10% due to heavy particles impact ionization. The ion mean energy close to the cathode is approximately reduced by in comparison with the value obtained by the reduced model with only the charge exchange collision considered. Charge production attributed to fast-neutral impact ionization not only causes the decrease of ion mean energy, but also enhances ion non-equilibrium.

Nesting-induced local electronic patterns around a single As (Te, Se) vacancy in iron-based superconductors

The local electronic states around a single As (Te, Se) vacancy are investigated in order to shed light on the role of ligands in a series of iron-based superconductors. Such a vacancy can produce a local hopping correction ranging from −0.22 to 0.12 eV and always induce two in-gap resonance peaks in the local density of states (LDOS) at the fixed symmetrical bias voltages, which are rather robust and irrelevant to the phase of superconducting order parameter. The LDOS images near the defect predominantly possess 0° and 45° stripes. These energy-dependent charge modulations created by quasiparticle interference are originated in the nesting effect between the inner (outer) hole Fermi surface around Γ point and the inner (outer) electron Fermi surface around M point.

Bottom-up engineering of thermoelectric nanomaterials and devices from solution-processed nanoparticle building blocks.

The conversion of thermal energy to electricity and vice versa by means of solid state thermoelectric devices is extremely appealing. However, its cost-effectiveness is seriously hampered by the relatively high production cost and low efficiency of current thermoelectric materials and devices. To overcome present challenges and enable a successful deployment of thermoelectric systems in their wide application range, materials with significantly improved performance need to be developed. Nanostructuration can help in several ways to reach the very particular group of properties required to achieve high thermoelectric performances. Nanodomains inserted within a crystalline matrix can provide large charge carrier concentrations without strongly influencing their mobility, thus allowing to reach very high electrical conductivities. Nanostructured materials contain numerous grain boundaries that efficiently scatter mid- and long-wavelength phonons thus reducing the thermal conductivity. Furthermore, nanocrystalline domains can enhance the Seebeck coefficient by modifying the density of states and/or providing type- and energy-dependent charge carrier scattering. All these advantages can only be reached when engineering a complex type of material, nanocomposites, with exquisite control over structural and chemical parameters at multiple length scales. Since current conventional nanomaterial production technologies lack such level of control, alternative strategies need to be developed and adjusted to the specifics of the field. A particularly suitable approach to produce nanocomposites with unique level of control over their structural and compositional parameters is their bottom-up engineering from solution-processed nanoparticles. In this work, we review the state-of-the-art of this technology applied to the thermoelectric field, including the synthesis of nanoparticles of suitable materials with precisely engineered composition and surface chemistry, their combination and consolidation into nanostructured materials, the strategies to electronically dope such materials and the attempts to fabricate thermoelectric devices using nanoparticle-based nanopowders and inks.

Charge of a quasiparticle in a superconductor

Nonlinear charge transport in superconductor-insulator-superconductor (SIS) Josephson junctions has a unique signature in the shuttled charge quantum between the two superconductors. In the zero-bias limit Cooper pairs, each with twice the electron charge, carry the Josephson current. An applied bias VSD leads to multiple Andreev reflections (MAR), which in the limit of weak tunneling probability should lead to integer multiples of the electron charge ne traversing the junction, with n integer larger than 2Δ/eVSD and Δ the superconducting order parameter. Exceptionally, just above the gap eVSD ≥ 2Δ, with Andreev reflections suppressed, one would expect the current to be carried by partitioned quasiparticles, each with energy-dependent charge, being a superposition of an electron and a hole. Using shot-noise measurements in an SIS junction induced in an InAs nanowire (with noise proportional to the partitioned charge), we first observed quantization of the partitioned charge q = e*/e = n, with n = 1-4, thus reaffirming the validity of our charge interpretation. Concentrating next on the bias region eVSD ~ 2Δ, we found a reproducible and clear dip in the extracted charge to q ~ 0.6, which, after excluding other possibilities, we attribute to the partitioned quasiparticle charge. Such dip is supported by numerical simulations of our SIS structure.

Maximizing thermoelectric properties by nanoinclusion of γ-SbTe in Sb2Te3 film via solid-state phase transition from amorphous Sb–Te electrodeposits

In this work, we demonstrate an enhancement of thermoelectric properties by creating γ-SbTe/Sb2Te3 nanocomposite film, where γ-SbTe nanoinclusions are embedded in a nanocrystalline Sb2Te3 matrix. The two-phase nanocomposite was formed via solid-state phase transition using an amorphous Sb2Te3 electrodeposits as the starting materials. The crystallinity and crystal structure of intermediate states during the amorphous-crystalline solid-state transformation were characterized by sequentially annealing the sample. The formation of the γ-SbTe is attributed to the different enthalpy of mixing for the bonding structures available in the Sb–Te system. Room temperature measurement of electrical and thermoelectrical properties as a function of annealing temperature revealed that the two-phase system provided the carrier energy filtering effect at the interfaces between two phases, leading to enhanced Seebeck coefficient without affecting its electrical transport properties. The band bending at the two-phase interfaces was indirectly manifested by measuring their difference in the valence band, advocating the possibility of a strong energy-dependent charge scattering to the enhanced Seebeck coefficient.

Resonance Raman and excitation energy dependent charge transfer mechanism in halide-substituted hybrid perovskite solar cells.

Organo-metal halide perovskites (OMHPs) are materials with attractive properties for optoelectronics. They made a recent introduction in the photovoltaics world by methylammonium (MA) lead triiodide and show remarkably improved charge separation capabilities when chloride and bromide are added. Here we show how halide substitution in OMHPs with the nominal composition CH3NH3PbI2X, where X is I, Br, or Cl, influences the morphology, charge quantum yield, and local interaction with the organic MA cation. X-ray diffraction and photoluminescence data demonstrate that halide substitution affects the local structure in the OMHPs with separate MAPbI3 and MAPbCl3 phases. Raman spectroscopies as well as theoretical vibration calculations reveal that this at the same time delocalizes the charge to the MA cation, which can liberate the vibrational movement of the MA cation, leading to a more adaptive organic phase. The resonance Raman effect together with quantum chemical calculations is utilized to analyze the change in charge transfer mechanism upon electronic excitation and gives important clues for the mechanism of the much improved photovoltage and photocurrent also seen in the solar cell performance for the materials when chloride compounds are included in the preparation.

OBSERVATION OF HIGH IRON CHARGE STATES AT LOW ENERGIES IN SOLAR ENERGETIC PARTICLE EVENTS

The ionic charge states of solar energetic particles (SEPs) provide direct information about the source plasma, the acceleration environment, and their transport. Recent studies report that both gradual and impulsive SEP events show mean iron charge states Q Fe ~ 10-14 at low energies E ≤ 0.1 MeV nuc–1, consistent with their origin from typical corona material at temperatures 1-2 MK. Observed increases of Q Fe up to 20 at energies 0.1-0.5 MeV nuc–1 in impulsive SEPs are attributed to stripping during acceleration. However, Q Fe > 16 is occasionally found in the solar wind, particularly coming from active regions, in contrast to the exclusively reported Q Fe ≤ 14 for low energy SEPs. Here we report results from a survey of all 89 SEP events observed with Advanced Composition Explorer Solar Energetic Particle Ionic Charge Analyzer (SEPICA) in 1998-2000 for iron charge states augmented at low energy with Solar and Heliospheric Observatory CELIAS suprathermal time-of-flight (STOF). Nine SEP events with Q Fe ≥ 14 throughout the entire SEPICA and STOF energy range have been identified. Four of the nine events are impulsive events identified through velocity dispersion that are consistent with source temperatures ≥2 MK up to ~4 MK. The other five events show evidence of interplanetary acceleration. Four of them involve re-acceleration of impulsive material, whose original energy dependent charge states appear re-distributed to varying extent bringing higher charge states to lower energy. One event, which shows flat but elevated Q Fe ~ 14.2 over the entire energy range, can be associated with interplanetary acceleration of high temperature material. This event may exemplify a rare situation when a second shock plows through high temperature coronal mass ejection material.

Super-Resolution Mapping of Photogenerated Electron and Hole Separation in Single Metal–Semiconductor Nanocatalysts

Metal-semiconductor heterostructures are promising visible light photocatalysts for many chemical reactions. Here, we use high-resolution superlocalization imaging to reveal the nature and photocatalytic properties of the surface reactive sites on single Au-CdS hybrid nanocatalysts. We experimentally reveal two distinct, incident energy-dependent charge separation mechanisms that result in completely opposite photogenerated reactive sites (e(-) and h(+)) and divergent energy flows on the hybrid nanocatalysts. We find that plasmon-induced hot electrons in Au are injected into the conduction band of the CdS semiconductor nanorod. The specifically designed Au-tipped CdS heterostructures with a unique geometry (two Au nanoparticles at both ends of each CdS nanorod) provide more convincing high-resolution single-turnover mapping results and clearly prove the two charge separation mechanisms. Engineering the direction of energy flow at the nanoscale can provide an efficient way to overcome important challenges in photocatalysis, such as controlling catalytic activity and selectivity. These results bear enormous potential impact on the development of better visible light photocatalysts for solar-to-chemical energy conversion.

A multi-energy (2–60 keV) calibration of200 μ m and 400 μm diameter spectroscopic GaAs X-ray photodiodes

Thin (2 μm active layer) spectroscopic p+-i-n+ GaAs X-ray photodiodes of circular mesa geometry (200 μm and 400 μm diameter; one representative diode of each diameter) have been characterised for their energy response using high-purity X-ray fluorescence calibration samples excited by an X-ray tube, giving energies between 2.1 keV (Au Mα1) and 21.18 keV (Pd Kα1), and an 241Am radioisotope γ-ray source (26.3 keV, 59.5 keV). The photodiodes were operated uncooled at +33°C. The 200 μm diameter device's energy resolution (FWHM) was found to be constant (0.79 keV) and primarily limited by electronics noise at energies between 2.1 keV and 21.18 keV, but it broadened to 0.85 keV at 26.3 keV, and to 1 keV at 59.5 keV. The 400 μm diameter device's energy resolution (FWHM) was constant (1.1 keV) for photon energies between 4.95 keV and 9.89 keV, but increased to 1.15 keV at 16.62 keV, 1.25 keV at 21.18 keV, 1.3 keV at 26.3 keV and 1.66 keV at 59.5 keV. The broadening of energy resolution (FWHM) observed in both cases is greater than can be attributed solely to increasing Fano noise and is hypothesised to be at least in part due to energy dependent charge trapping. However, for both types of device, the peak charge output from the devices was found to be linearly (R2 ⩾ 0.9999) dependent on incident X-ray energy.