Ferroelectrics are materials with a spontaneous electric polarization that can be switched by applying an electric field. 
The direction of the ferroelectric polarization is typically not uniform throughout a crystal but instead forms domains with different polarization orientations. 
These domains either form spontaneously, or they are created intentionally.
The interfaces between the domains, the ferroelectric domain walls, can be considered as planar structural defects, whose behavior may differ from that of the host crystal. For example, in materials like lead zirconate titanate or bismuth ferrite, which are normally insulating or semiconducting, a significant increase in the electrical conductivity has been observed at ferroelectric domain walls [1,2]. 
Within the domain-wall planes, additional point-like defects may further change the behavior of the domain wall. 
In my talk I will elucidate how atomic defects and charge carriers behave at ferroelectric domain walls in bismuth ferrite, according to ab initio calculations, and how they could be detected in optical spectra.
More precisely, I will present evidence that excess electrons in bismuth ferrite accumulate in strongly localized small polaron states at ferroelectric domain walls, where they should either recombine with holes under emission of photoluminescence with photon energies below the band gap energy or participate in electronic transport by thermally activated hopping of small electron polarons. 

[1] J. Guyonnet, I. Gaponenko, S. Gariglio, and P. Paruch, Conduction at domain walls in insulating Pb(Zr0.2Ti0.8)O3 thin films, Adv. Mater. 23, 5377 (2011).
[2] J. Seidel, L. W. Martin, Q. He, et al., Conduction at domain walls in oxide multiferroics, Nat. Mater. 8, 229 (2009).

Deterministic implantation of single ions is currently of high interest for quantum technology applications. We have recently commissioned two instruments to carry out deterministic implantation across a range of ion species. These systems located in the UK National Ion Beam Centre (UKNIBC) are bespoke focussed ion beam systems designed specifically for the task of deterministic implantation and comprise one, which has a liquid metal ion source, and the second, a duoplasmatron source. Both systems use a pulsed, low current beam of ions and contain Wein filter mass/charge filtering; neutral filtering and secondary electron detection to determine when an implantation event has occurred. They operate an EBL-like strategy for precise location of the ion pulse at the desired locations and with resolutions on the same scale as the ion straggle.

While we have now run across the two systems a range of sources such as In, Bi, Si, C, N, O, Ge, Er, Sn and Cr the task ahead is to establish stable and reliable sources for additional species (for example rare earths) of particular interest to the quantum research community and we have an ongoing sources development program to expand the range of available species. Here we also present single ion implant detection results from the single ion implanter where we study detection efficiencies for different ion species/substrate combinations.

A fundamental understanding of electron-phonon interaction and underlying electron dynamics in nano-electro-mechanical resonators remains limited yet imperative towards the control and detection of their high-frequency motion. Herein, we introduce an atomically resolved shot-noise study of an atomically sized nano-electro-mechanical system, consisting of an Fe impurity coupled to the phonon mode of the Bi₂Se₃ host lattice. We observe strong signatures of phonon-assisted tunnelling that can be explained within the Franck-Condon framework. In addition, the system displays super-Poissonian noise, inferring the presence of electron dynamics during the tunnelling process through the resonator. With the aid of a theoretical model, we show that the excess noise observed in the experiments arises from a mechanical oscillation of the impurity between the electrodes resulting in electron bunching. This effect overwhelms the typical Coulomb blockade-induced sub-Poissonian noise observed on impurities in the same material that do not couple to the phonon mode. Apart from the fundamental physics, our findings also suggest that measuring shot noise could potentially serve as a crucial element for the detection of quantized mechanical motion.

Historically, research in semiconductors has been aimed at the removal of defects because of their negative effect on the performances of devices. In recent years, researchers have embraced the presence of defects in materials, whether intrinsic or inserted by design. Defects have started to be considered as core elements of innovative devices in frontier applications such as quantum technologies, acting for example as single-photon emitters or quantum memories. These applications might require operation in the infrared, but since most known platforms consist of defects in wide-bandgap semiconductors, systems working in this spectral range are still scarce. This is particularly true for operation in the short-wave infrared. A potentially promising material system for this scope is that composed of Ge-on-Si heterostructures such as Ge/SiGe multiple quantum wells (MQWs). These present extended defects that can be optically active and emit light at around 2 µm. The emission properties of defects in Ge/SiGe MQWs can be of great interest for mid-infrared photonic applications, but the characterization of such defects is still at a seminal stage.

In this work we characterize by means of optical spectroscopy the photoluminescence (PL) of extended defects in p-i-n diodes composed of Ge/SiGe MQWs. The PL spectrum of unbiased devices is dominated by the defect line at 2 µm. Surprisingly, the defect line emits circularly polarized light with a degree of polarization of about 8% at 5K. This result implies that the spin coherence lifetime of carriers trapped on the extended defects is longer than the carrier lifetime, thus suggesting the use of extended defects in spintronics. We also show how the degree of polarization of the defect emission can be modified by changing easily tunable experimental parameters, namely the lattice temperature, the excitation power density, and the external applied bias.

The possibility of exploiting intrinsic defects in group-IV based heterostructures for spintronics is particularly exciting, and the control of the polarization degree through readily available turning knobs such as an applied voltage, readily suggests the use of such defects for emerging applications as low-energy polarized emitters.

β-Ga2O3 is a novel functional material of an exceptionally wide bandgap (4.7 eV), which provides a route to applications as a semiconductor in power and optoelectronics, as well as batteries. Still, quite challenging is to conserve the monoclinic phase of β-Ga2O3 that undergoes nanostructuring without development of parasitic phase polymorphs. The control over different Ga₂O₃ crystal phases can be realized using ion irradiation, particularly employing noble gases, like Ne+. In this study, a phase transition from β to γ has been observed as a function of ion fluence and investigated by positron annihilation spectroscopy (PAS). A peculiar evolution of defect microstructure has been captured using Doppler broadening PAS, where an accumulation of defects is observed for a low fluence range, followed by a sudden reduction of defect density in the range of ~10¹⁶ ion·cm^-2, with further increase for larger fluences. The reduction of defect concentration gives an onset of the transition to the γ-phase. Utilizing the pulsed positron beam at ELBE an estimation of positron lifetimes during the phase transition was possible. At the threshold fluence, positron lifetime raises from a value expected for a Ga vacancy (V_Ga) in the β-phase [1] to a dimension predicted for V_Ga in the γ-phase, according to our ab-initio DFT calculations. The γ-phase is characterized by a high level of disordering and no clear free positron state, in contrast to the β-phase, hence the observed drop of positron lifetime for the largest fluence reflects most probably the intrinsic modifications of the disorder magnitude.

[1] A. Karjalainen, V. Prozheeva, K. Simula, I. Makkonen, et al., Phys. Rev. B. 102, 195207 (2020).

As an air-stable Van der Waals magnetic semiconductor, CrSBr is receiving great research attention due to its exceptional optical, electronic, and magnetic properties. Below the Néel temperature of 132 K, CrSBr exhibits a typical A-type antiferromagnetic order comprised of antiferromagnetically coupled ferromagnetic monolayer. This special structure makes it susceptible to external stimuli, such as defects. In this work, we present the magnetic phase transition from antiferromagnetic to ferromagnetic in CrSBr crystals irradiated by non-magnetic ions. We observe the rise and fall of the ferromagnetic phase in antiferromagnetic CrSBr with increasing the irradiation fluence. The irradiated CrSBr shows ferromagnetic critical temperature ranging from 110 to 84 K, well above liquid N2 temperature. The induced ferromagnetism shows the easy-axis anisotropy along the b-axis. Structure analysis of the irradiated crystals in conjunction with density functional theory calculations suggest that the displacement of constituent atoms due to collisions with ions and the formation of interstitials favors ferromagnetic order between the layers.

[1] F. Long, K. Mosina, R. Hübner, et al. Intrinsic magnetic properties of the layered antiferromagnet CrSBr[J]. Applied Physics Letters, 2023, 123(22).

[2] E.J. Telford, A.H. Dismukes, K. LeE, et al. Layered antiferromagnetism induces large negative magnetoresistance in the van der Waals semiconductor CrSBr[J]. Advanced Materials, 2020, 32(37): 2003240.

[3] F. Long, M. Ghorbani-Asl, K. Mosina, et al. Ferromagnetic interlayer coupling in CrSBr crystals irradiated by ions[J]. Nano Letters, 2023, 23(18): 8468-8473.

[4] F. Long, Y. Li, Y. Cheng, et al. Rise and fall of the ferromagnetism in CrSBr flakes by non-magnetic ion irradiation[J]. Advanced Physics Research, 2024, accepted.

On many occasions, structural defects bare a negative connotation, provoking efforts to reduce their contents. On the other hand, many unique material properties may become possible only with defects in materials. Moreover, introduction of defects in a controlable way may induce very interesting phase transformations on nanoscale, as it was recently demonstrated in Ga2O3 polymorphs (Physical Review Letters 128, 15704 (2022)). Specifically, we showed that even though the thermodynamically stable monoclinic polymorph (beta-Ga2O3) can be swiftly disordered, it does not amorphize under irradiation, but converts into a cubic defective spinel polymorph (gamma-Ga2O3). Concurrently, the rationale behind this remarkable beta-to-ggamma Ga2O3 polymorph flipping is because the oxygen sublattice in these polymorphs, exhibiting face-centered cubic structure, demonstrates strong recrystallization trends, while the Ga sublattice is susceptible to disorder (Nature Communications 14, 4855 (2023)). Very recently, this idea was exploited to demonstrate multiple gamma/beta polymorph repetitions by adjusting spatial distributions of the disorder levels as a function of the irradiation temperature and ion flux (Physical Review Letters, in-review (2024) and arXiv:2401.07675v3); as such we demonstrated defect-induced nano-engineering of “polymorph heterostructures” not being realized by conventional growth methods otherwise.

For next generation energy and electronic device applications, piezoelectric, pyroelectric, and ferroelectric materials are of high technological and industrial importance due to their energy conversion properties. Piezoelectricity converts mechanical energy to electrical energy, and vice versa, while pyroelectricity utilizes temporal changes in temperature-dependent spontaneous polarization for electricity generation. These phenomena have been extensively investigated over the centuries for their applications such as actuation, sensing, energy harvesting, and thermal management. However, within the current research paradigm, the choice of piezoelectric and pyroelectric materials is fundamentally limited to those with non-centrosymmetric crystal structures. Besides the existing strategies for material design and engineering, controlling ionic defects such as atomic vacancies can be an effective approach for modifying material crystal symmetry/dimension, enabling the generation of new functional properties including emergent piezoelectricity and pyroelectricity. In this talk, I will introduce how the fundamental crystal limitation can be circumvented by breaking the lattice symmetry of intrinsically centrosymmetric oxide materials [e.g., Gd-doped CeO_2-x (CGO) and undoped SrTiO₃ (STO)] to induce emergent electromechanical coupling and pyroelectricity [1-3]. These can be achieved by creating, controlling, and stabilizing atomic charge defects using various methods, including electric field manipulation and thin film growth techniques. Such defect engineering leads to the generation of sustainable and highly efficient piezoelectric and pyroelectric effects in the engineered CGO and STO thin films, which are comparable with or even beyond those of current high-performance piezoelectric and pyroelectric materials. Furthermore, the unprecedented defect-mediated phase separation and domain structure formation observed in these film materials will be discussed. 

References

[1] D.-S. Park, et al., Induced giant piezoelectricity in centrosymmetric oxides Science 375 653-657 (2022).

[2] A. D. Rata, et al., Defect-induced magnetism in homoepitaxial SrTiO₃. APL Mater. 10, 091108 (2022).

[3] D.-S. Park, et al., Tunable pyroelectricity in centrosymmetric oxides. In preparation (2024).

Defect engineering in functional oxide materials has been used as an effective way to tune the physical properties of oxide materials. Fluorite-based crystalline materials have the general formula AX₂ (A = Ca, Hf, Ce, Zr, while X = F, O) and find applications in many energy systems. Despite their relatively simple centrosymmetric structure, fluorites are still the subject of extensive research and discoveries,[1] such as ferroelectricity in HfO₂-based thin films or large electromechanical coupling in Gd-doped CeO_2-x (CGO).[2] Recent work demonstrates the possibility of inducing piezoelectric effects from nominally centrosymmetric CGO by evoking a defect-mediated symmetry-breaking through the control of ionic charged defects, specifically oxygen vacancies (V_O).[2] Furthermore, the study revealed that redistributing V_O through the application of an electric field is crucial for controlling the dielectric permittivity and electromechanical properties of systems with oxygen deficiencies. Here, we demonstrate an effective method for controlling the V_O contents of epitaxial CGO films (~100 nm thick) grown on Nb:STO(001) single crystals using pulsed laser deposition. Our results show that the control of V_O content is challenging in the CGO films during the high-temperature (e.g., T_g = 700 °C) film growth. To overcome this, we propose a two-step film growth, i.e., an initial high-temperature (e.g., T_g = 700 °C) buffer layer growth, followed by a low-temperature (e.g., T_g = 400 °C) film growth.[3]  This method allows for effective tuning of the V_O content in the CGO films. This approach subsequently allows for generating giant apparent dielectric permittivity (e.g., the real part of permittivity, ε´ ≈ 10⁶) under the control of electric field application. This work offers important insights for potentially inducing large electromechanical coupling in the CGO film materials.[4]

Reference

[1] U. Schroeder, et al., Nat. Rev. Mater., 7(8), 653–669 (2022).
[2] D.-S. Park, et al., Science, 375, 653–657 (2022).
[3] A. Palliotto, J. Phys. Energy, 6, 025005 (2024).
[4] D. Damjanovic, Rep. Prog. Phys., 61(9), 1267–1324 (1998).

SnS is a p-type thermoelectric material, which is composed of abundant and relatively non-toxic elements. Using nanoparticles (NPs) as a building blocks in thermoelectric materials is known to reduce thermal conductivity resulting in improved performance. To shine light on the mechanisms involved in the doping of the sustainable thermoelectric material SnS with Ag and Se, we present a detailed investigation into Ag- and Se-doped SnS NPs. SnS NPs, Ag-doped SnS NPs, and Ag-doped SnS_1–xSe_x NPs were chemically synthesized and sintered into pellets by hot-press. The structure and thermoelectric, electronic, and thermal transport properties were then investigated using a variety of techniques. As a result, it was found that when Ag-doped SnS NPs were sintered two types of Ag were present in the sintered pellets: one in the form of segregated Ag-rich nanoprecipitates and the other in the form of interlayer intercalated Ag ions. In contrast, when Ag-doped SnS_1–xSe_x NPs were sintered, Se was found to form a homogeneous solid solution. The effects of these three impurity-derived structures on the electronic and thermal transport properties were investigated. The final ZT values for SnS doped with 1.5 at% Ag (SnS:Ag) and SnS_0.9Se_0.1:Ag, in which SnS:Ag was further doped with Se, were 0.09 at 666 K and 0.14 at 667 K, respectively.

Iso-electronic dopants in III-V semiconductors can introduce interesting defect states that are applied at the single defect level. However especially iso-electronic dopants causing a large lattice mismatch are sometimes difficult to incorporate properly. In this contribution we will present an STM study of iso-electronic dopants where we explore their electronic properties and analyse their pairing and clustering behaviour at the single defect level. We will focus on N, Bi and Sb in GaAs and InAs as exemplary iso-electronic dopants that give rise to interesting defect states and/or are know for their difficulty to create high-quality materials.

The band gap of most III-V semiconductors is strongly reduced with the introduction of only a few percent of N. We used STM to obtain spatial images and spectroscopic data on N atoms up to two layers below the surface of InAs. Spatial imaging of N dopants up to two layers below the surface are compared to density functional theory simulations and show excellent correspondence. Spectroscopy maps of N atoms showed an enhancement of the dI/dV signal compared to the InAs background. At energies above the enhancement a reduction of the dI/dV is observed showing that the redistribution of density of states caused by the N atoms is mainly energetic in nature.

Introducing only a few atomic percent of Bi or N in GaAs has a large effect on the band gap. The incorporation of Bi and N into GaAs is however difficult due to strain effects. We studied the ordering of these dopants at the atomic scale to get an understanding of their incorporation in GaAs. STM allows to determine the exact position of Bi and N dopants in the GaAs matrix, allowing us to study both the nearest neighbour pair occurrences and pair correlation functions. An attractive interaction between Bi dopants at short ranges is found and a similar effect is observed between N dopants. We find a repulsive interaction with a similar length scale between Bi and N dopants. Density functional theory (DFT) is used to calculate the different nearest neighbour pair energies. Our experimental and theoretical results show that the exact growth conditions affect the Bi and N distribution in GaAs.

GaAsSb is a challenging material to grow because of As-Sb exchange and a tendency to accumulate on the growth surface. Moreover, Sb dopants can have the tendency to cluster which might negatively influence the optical and electronic properties of the material. Sb atoms up to five layers below the surface are visible in the STM images. These features are classified and related to their depth with the help of density functional theory calculations. Sb incorporation and segregation are studied, and we observe that under the employed growth condition Sb appears to be rapidly incorporated and to have a rather limited a tendency to segregate. Additionally, we found that Sb does not have a tendency to form pairs or clusters.

Microstructure engineering is widely used to customize material properties. These include thermal properties, such as material thermal conductivity, which is crucial in applications ranging from thermal management to energy harvesting. Understanding and controlling the impact of extended phonon-scattering defects, like grain boundaries, on thermal conductivity is essential for efficient material design, yet systematic studies are limited by the lack of adequate tools. This study demonstrates an approach for measuring grain boundary thermal resistance by probing thermal wave propagation across grain boundaries. Thermal waves are generated in a sample using a lithographically defined microheater, and the resulting thermal wave field is mapped with a temperature-sensitive scanning probe. This approach allows for highly localized point-by-point measurements of the thermal wave field and simplifies data analysis. We have implemented the method with a spatial resolution of about 100 nm on Nb-substituted SrTiO₃ ceramics with an average grain size of approximately 5 μm. Using a linearized analytical model and numerical simulations, we quantified grain boundary thermal resistance by assessing changes in thermal wave amplitude and phase across grain boundaries in the ceramic. Detectability of approximately 2×10⁻⁸ K m² W⁻¹ was achieved, which makes this method directly applicable to chalcogenide-based thermoelectric materials, where typical grain boundary thermal resistances are on the order of 10⁻⁸ K m² W⁻¹. In turn, grain boundaries in oxides, with lower thermal resistances, require higher detectability. Our measurements indicated that most grain boundaries in STiO₃ ceramics have thermal resistance below this value. A major challenge in enhancing detectability is the probe's sensitivity to variations in the thermal resistance at the probe-sample contact. These variations stem from the relatively low thermal resistance between the probe and environment, coupled with large thermal resistance at the probe-sample interface. Future advancements can mitigate these effects through improved signal-generation techniques. Despite this, considering the spatial resolution and the amount of material involved in the detection, our method's sensitivity is at least comparable to optical thermoreflectance methods. This method enables the characterization of thermal resistance at the level of single grain boundaries, domain walls, and other defects in microstructured materials.

The work at the Univ. of Aveiro was supported by FCT/MEC (POCI-01-0145-FEDER-032117) and FCT/MCTES through CICECO (UIDB/50011/2020, UIDP/50011/2020 & LA/P/0006/2020). AT acknowledges the 2021.03599.CEECIND contract through FCT. WX and AW acknowledge support by DAAD (Project-ID: 57610929). AA, EP, and MC thank the Center “Modern nanotechnology” for access to equipment. In part this research was a user project at the CNMS, which is a US DOE OS User Facility at ORNL.

Fluorine-doped barium stannate nanoparticles (BaSnO₃:F) were synthesized using a previously developed solution combustion synthesis method[1]. The addition of the fluorine source to the peroxo precipitates, followed by annealing in a reducing environment, not only yields phase-pure BaSnO₃ but also produces an optical response similar to that observed with typical dopants at the cation site. X-ray photoelectron spectroscopy (XPS) and STEM-EDS are employed to confirm the presence of fluorine in the synthesized nanoparticles and pressed pellets. The XPS confirms two types of fluorine in the lattice, with one type increasing in concentration with the doping and the other remaining constant. Based on the binding energy of these fluorine species and their effect on the binding energies of the Ba and Sn cations, we hypothesize the location of the fluorine atoms and the defects in the system. Furthermore, Hall measurements carried out on the pellets confirm n-type conductivity with mobility in the range of approximately 10¹ cm²V⁻¹s⁻¹, but with significantly reduced carriers as expected due to complete ionization of the added fluorine. We also discuss the role of pores, grain boundaries, and compensating defects leading to this observation.

[1] Chawla, S., Aggarwal, G., Kumar, A., Singh, A. J., Woodward, P. M., & Balasubramaniam, K. R. (2024). Low-temperature synthesis of transparent conducting La-doped BaSnO₃ via rejuvenation of the dried peroxo-precursor. Journal of Solid State Chemistry, 333, 124620.

 Hydrogen production directly from water is the efficient source for green, environmentally friendly energy. Sunlight-driven water splitting is one of the most promising pollution-free strategies for production of hydrogen. Photocatalytic water splitting consists of water decomposition into hydrogen and oxygen by a reaction with photo-generated charge carriers. However, many challenges must be overcome before photocatalytic water splitting can be practically implemented at a large scale.

We discuss the results of large scale first-principles calculations on structural and electronic properties of SrTiO3 (STO) perovskite photocatalyst (band gap 3.25 eV) and how to modify its electronic band structure by means of defects and impurities. DFT calculations were performed with CRYSTAL17 computer code within the linear combination of atomic orbitals (LCAO) approximation and using B1WC advanced hybrid exchange-correlation functional. We considered the bulk STO crystal and its (001) 2D slabs, as well as faceted nanoparticles. A supercell was used to simulate point defects (neutral and charged oxygen vacancies, N and Al substitutional atoms [1-4]). Introduction of these defects indeed makes STO photocatalyst more efficient for sunlight-driven water splitting. 1. L.L. Rusevich, M. Tyunina, E.A. Kotomin .Scientific Reports., 2021, 11, 23341 (pp. 1-8). 2. Y.-Y. Tai, J.C.S. Wu, W.-Y. Yu, M. Maček Kržmanc, E.A. Kotomin. Appl. Catal. B, 2023, 324, 122183 3. M.E. M. Sokolov, Yu.A. Mastrikov, E.A. Kotomin et al, MDPI Catalysts, 2021, 11, 1326 (pp. 1-8) 4. M. Sokolov, Yu.A. Mastrikov,  E.A. Kotomin et al. Catal. Today 2024, 432, 114609 (pp. 1-7).

Near-field microwave microscopy is a scanning probe technique exploiting electric fields oscillating at a GHz-frequency formed at a sharp scanning probe in contact with a sample. The technique allows mapping and localized quantitative characterization of material dielectric permittivity and conductivity. The spatial resolution can be as high as few tens of nm. It this talk, we introduce the technique and show its application to quantitative characterization of AC conductivity of domain walls formed in ferroelectric thin films. We show that spontaneous and artificially injected domain walls in thin films of Pb(Zr0.2Ti0.8)O3 demonstrate a large conductance at GHz frequencies being insulating at DC. The use of a GHz frequency makes the probing nearly insensitive to the high electrical resistance of the interfacial barrier between the probe and the sample that prevents meaningful probing at low-voltage DC. This feature open the door to the use of small electric fields for probing the domain wall conduction in ambient without disturbing the ferroelectric domain structure. The work at the Univ. of Aveiro was supported by FCT/MEC (POCI-01-0145- FEDER-032117) and FCT/MCTES through CICECO (UIDB/50011/2020, UIDP/50011/2020 & LA/P/0006/2020). The author acknowledges support by the 2021.03599.CEECIND contract through FCT.

During the last decade Gallium oxide (Ga2O3) has emerged as a viable candidate for a number of power electronic and optoelectronic devices with potentiality to exceed existing technology based on well-established wideband gap semiconductors.  This ultra-wide bandgap semiconductor with room temperature bandgap around ~4.8 eV (reported in the range 4.6-4.9 eV) has five different polymorph, namely,  rhombohedral (α), monoclinic (β), cubic (δ), defective spinel (γ), or orthorhombic (e) structures.  Among them, the b-polymorph phase is the most thermodynamically stable and bulk crystals have been successfully grown from various melting methods. As result, the b-Ga2O3 polytype is the most widely studied and utilized on the fabrication of testing devices. This semiconductor is typically semi-insulator or n-type conductor, depending on the concentration of intrinsic point defects (oxygen or gallium vacancies) and/or background impurities. The goals of this work is to understand the deposition of high quality β-phase Ga2O3 films by Ion Beam Assisted Deposition (IBAD) method and its potential application as optoelectronic sensor. Our research was focused on the deposition of high quality β-phase Ga2O3 thin films on Si substrates at room temperature with optimize deposition conditions to produce films with improved structural, optical, and electronic properties. The Ga2O3 b-phase was verified by XRD analysis, while the bandgap value was obtained using the Tauc method. Evaporated aluminium films at the top and bottom surfaces of the templates were employed as device contacts. A probed device structure is depicted in Figure 1a, which is comprised of four layers: Al/Si/Ga2O3/Al. RBS analyses indicate that thin silicon oxide interface layers were formed on p-type Si surface during the IBAD deposition process, which may results from oxygen ion beam falling on the substrate prior gallium oxide film deposition. There is still a process of formation of Ga2O3 doped with aluminium at the interface of the Al contacts and the insulating Ga2O3 layer. Current versus voltage (IxV) measurements were obtained using an HP 4140B picoammeter. Data acquired under light soaking, using a halogen lamp, will be presented. The devices showed photoelectric effect and Schottky behaviour.

Modern synthesis techniques and advanced synchrotron characterization, combined with computational methods, have enabled precise design of metal atom environments by controlling oxygen vacancies. In this study, we present cobalt (Co) single-atom-incorporated titanium dioxide (TiO2) monodispersed nanoparticles synthesized through a thermodynamic redistribution strategy. Synchrotron-based X-ray techniques and simulations reveal the absence of trivalent titanium (Ti3+), indicating that these cations do not affect ferromagnetic stability. Density functional theory (DFT) calculations show weak ferromagnetic stability between Co²⁺ ions alone. However, electron doping from additional oxygen vacancies significantly enhances this stability, explaining the observed room-temperature ferromagnetism. Our calculations further demonstrate stronger ferromagnetic interactions between CoTi + VO complexes with more oxygen vacancies. This study is the first to explore the electronic structure and room-temperature ferromagnetism in monodispersed nanocrystallites with single-atom-incorporated TiO2 nanostructures. These findings provide a new strategy for uncovering magnetism in other single-atom-incorporated nanostructures. This research was supported by the IBS in Korea, NRF, ESRF, and Yale.

Emerging memory devices play a major role in implementing artificial neural networks as basic types of neuromorphic hardware. They provide extra functionality to conventional CMOS technology, such as the ability to implement non-volatile memory within a nanoscale region on the chip. Resistive switching random access memory (RRAM) devices are very promising for these applications. The mechanisms of resistance changes have generally been attributed to defect generation and aggregation in the oxide and depend on the chemical nature of metal electrodes. Using multiscale modelling, we investigate the role of electron injection and hydrogen incorporation inside amorphous (a) oxide films of SiO₂, HfO₂, Al₂O₃ and at interfaces with Si and TiN electrodes in creation of new defects, oxide degradation, and resistance change. The initial models of amorphous structures are created using classical force-fields and the LAMMPS package. The volume and geometry of all structures are fully optimized using density functional theory (DFT) implemented in the CP2K code with the range-separated hybrid PBE0-TC-LRC functional, as described in detail in [1]. The results demonstrate that extra electrons injected into oxide localize in amorphous SiO₂ and HfO2 in deep states about 3.0 eV below the mobility edge [2]. Trapping of up to two electrons at intrinsic sites results in weakening of Si-O and Hf-O bonds and emergence of efficient bond breaking pathways for producing neutral O vacancies and interstitial O_i^2- ions with low activation barriers [2]. These barriers as well as barriers for migration of the O_i^2- ion (< 0.5 eV) are further reduced by bias application. Simulations of SiO₂/TiN interfaces [3] explain how, as a result of electroforming, the system undergoes very significant structural changes with the oxide being significantly reduced, interface being oxidized, and part of the oxygen leaving the system. Creation of O vacancies facilitates trap-assisted tunnelling through oxide films and is responsible for oxide charging and leakage current. Hydrogen and metal incorporation from metal electrodes leads to creation of additional defects in the oxide. Atomistic simulations of defect creation in amorphous oxide films are combined with kinetic simulations of trap assisted tunnelling of electrons and ionic diffusion through oxide [4]. They provide the mechanisms and time evolution of oxide charging and degradation. These mechanisms are used to simulate the kinetics of electroforming and explain the mechanisms of set, reset and retention in RRAM devices.

[1] A-M. El-Sayed et al., Phys, Rev. B 89, 125201 (2014)

[2] J. Strand et al., J. Phys.: Condens. Matter 30, 233001 (2018)

[3] J. Cottom et al. ACS Appl. Mater. Interfaces 11, 36232 (2019)

[4] J. Strand et al. J. Appl. Phys. 131, 234501 (2022)

Herein, we present a study of the effects of native cadmium-indium sulfide (CdIn₂S₄) defects on its electronic properties, including conductivity switching. Our work shows that native CdIn₂S₄ defects alone are sufficient for switching applications and analog and neuromorphic computing. We also explore metastability-driven resistive switching mechanisms in semiconductors.

The CdIn₂S₄ - a n-type semiconductor has a spinel structure, where the sulfur atoms are arranged in a cubic close-packaged lattice forming tetrahedrals and octahedrals. In a normal spinel, Cd and In cations occupy centers of the tetrahedral and octahedral sites, respectively. In the inverted spinel structure, the cation positions are swapped. A fractional degree of inversion is possible, where only some cations are interchanged. The degree of spinel inversion directly affects its optical and electronic properties and is a source of native defects, i.e., In_Cd and Cd_In antisites. Ab initio simulations show that Cd_In and sulfur vacancy - another native defect in CdIn₂S₄ - form metastable complex states.

To investigate the material, we deposited CdIn₂S₄ thin films on soda lime glass substrates using co-evaporation. Then, the layers were subjected to post-deposition treatment, i.e., annealing in a vacuum or sulfur atmosphere. We performed a series of electrical measurements probing the mechanisms that govern the electrical behavior of the compound. The conductivity of the as-grown films is controlled by a high concentration of In_Cd states introduced by the initial inversion degree, which forms the distribution of sub-bandgap states and pins the Fermi level at 0.1eV below CB, resulting in a low compound resistivity. Annealing the layer in a vacuum leads to the relaxation of the spinel structure and, therefore, a reduction of the inversion degree and concentration of In_Cd states, then revealing the intricate behavior of the Cd_In&V_S complex, which now pins the Fermi level at 0.4eV below CB. Thus, the resistivity of the compound is increased. As the Cd_In&V_S complex is metastable, persistent photoconductivity phenomena occur. It is then possible to convert the charge state of the metastable defects by illuminating the compound with a particular wavelength. This results in repining the Fermi level and altering the free carrier concentration. Thus, by illuminating the layers, we can gradually control its resistivity, practically realizing the switching operation. Annealing the layers in sulfur further lowers the fermi level, showing the importance of the sulfur vacancy.

​​​By further tuning the stoichiometry of the compound, we can fine-tune the subtle balance between the In_Cd and Cd_In&V_S-controlled regimes, gaining more control of the phenomena. We performed a series of optoelectronic measurements in the function of temperature, incident light power, and wavelength, getting insight into the nature of the metastable defects.

Gallium oxide (Ga2O3), a transparent semiconductor material with reported wide band gap between 4.5-4.9 eV, has attracted increasing interest lately due to multifunctionality and bulk solution growth. In this work, thin Ga2O3 films were deposited by ion beam assisted deposition (IBAD) method on p-type silicon at ~800 °C. It was observed that the substrate temperature plays an important role on film crystalline phase formation. Films deposited at lower temperatures predominantly have β-phase structure, while those deposited at higher temperatures have dominant ε-phase, as verified by X-ray diffraction. RBS measurement indicates that a layer of 350 (±24) nm has been deposited. Additionally, intermediate silicon oxide layer have been introduced between the thin film and the substrate. This layer may have been formed during the initial stage of IBAD deposition, where silicon is bombarded by oxygen ion beam before gallium ions reaches the hot substrate. Memristors testing devices were fabricated by depositing aluminum contacts over the thin Ga2O3 film surface and the back surface of the Si substrate. The RBS structural analysis indicates that there was aluminum diffusion from the top contact into the thin Ga2O3 film. The current-voltage measurement of the isolated device provided key information for understanding the memristor operation, revealing its unipolar characteristic and responsiveness to visible light soaking. Therefore, the tested devices are, in fact, photomemristors. The filaments formation in the memristor most likely result from the existence of oxygen vacancies and/or ionic impurities (e.g., Si is a shallow donor impurity in Ga2O3). Oxygen vacancy is an intrinsic point defect in Ga2O3, but is not the unique defect responsible for n-type conductivity. The SiO2 layer, formed between the silicon substrate and the thin Ga2O3 film, observed through RBS analysis, could be an indication that Si also diffuse into the Ga2O3 film. In addition, aluminum diffused from the contacts into the Ga2O3 film may remove oxygen from the Ga2O3 film structure, considering that Al is an oxygen getter, increasing the oxygen vacancy concentration. In the proposed neuromorphic circuit measurement, we aim to use a circuit that emulates a neuronal membrane and study the behavior of memristors response to light stimulus. This approach has proven to be an excellent tool for observing action potentials, which mimic human vision.

Resistive-switching-based memory is a popular research area for majorly neuromorphic, nonvolatile memory design and in-memory computing. Pr1−𝑥⁡Ca𝑥⁢MnO3 [PCMO(𝑥)] is one of the most explored perovskite materials for resistive random access memory (RRAM) switching based on electric-field-induced charged oxygen vacancy (V^𝑞_O) migration. Thus PCMO⁡(𝑥 = 0,0.3,1) has been studied thoroughly on the experimental as well as theoretical front for resistive switching properties. However, the experimentally observed large variation (∼33%) in activation energies for the V𝑞O migration in these materials makes it difficult to understand the V^𝑞_O migration in PCMO(𝑥)-based RRAM. In this work, first-principles-based density functional theory (DFT) calculations are employed to calculate the stability and migration dynamics of the V^𝑞_O as observed in our experiments performed for PCMO(𝑥). Taking into account the electronic and structural relaxation, the spin polarization, and the effect of strong electronic correlation, DFT calculations are carefully benchmarked to reproduce the experimental lattice parameters and electronic band gap for the end members, i.e., for PrMnO3 (PMO) and CaMnO3 (CMO). The defect formation energy calculated for a single V^𝑞_O reveals the oxygen vacancies to be charged with V⁺²_O broadly remaining stable in PMO and CMO. The nudged elastic band calculations are performed for migrating V⁺²_O, which reveal activation energies 𝐸_mig,a of 0.96 and 0.83 eV for the migration of the charged oxygen vacancy in PMO and CMO [1], respectively, which is in the range of our experimentally observed 𝐸_mig,a. This provides much-needed insights for the fundamental understanding and modeling of the vacancy dynamics in PMO- and CMO-based RRAM devices.

[1] Shashank Inge, Adityanarayan Pandey, Udayan Ganguly and Amrita Bhattacharya, Phys. Rev. B, 108, 035114 (2023)

Graphene has great potential for use as a building block in a wide range of applications from nanoelectronics to biosensing and high-frequency devices due to its high electrical conductivity, low density, and flexibility. For many device applications, it is required to deposit dielectric materials on graphene at high temperatures and in ambient oxygen. This has been proven to be highly challenging because these conditions cause significant degradation in graphene.

This work will present a deposition method of metal oxide materials on graphene at high temperatures without significant degradation in graphene quality. We will demonstrate the possibility of manufacturing a bulk acoustic wave resonator (BAW) without a Bragg reflector and back-etched substrate and comment on the fabrication conditions and device performance.

In 2019 ^[1], a new family of superconducting thin films was discovered : nickelates. That property is report in lanthanide doped by strontium nickel infinite layers (Pr, Nd, La)^[2]. Infinite layers are synthesize by a topotactic reduction with calcium hybride of the perovskite phase^[3]. The theoretical structure^[1] with in plane NiO₂ could explain the superconducting property. The aim of this project is to understand the mechanism of the superconductivity in those compound and study the structure.

First step is to synthesize high qualities thin films of doped and undoped perovskite of Pr_1-xSr_xNiO₃ and Nd_1-xSr_xNiO₃ by pulsed laser deposition and reduce them in infinite layer phases Pr_1-xSr_xNiO₂ and Nd_1-xSr_xNiO₂. The X-ray fluorescence highlighted a deficit of nickel in our thin film and it could be link to transport properties (resistivity).

Infinite layers of Pr_1-xSr_xNiO₂ are synthesize and preliminaries measurements show a strong impact of the strain in the homogeneity of thin film. A study of transport properties of infinite layers of Pr_1-xSr_xNiO₂ evince more strain there is in the structure more the reduction process is inhomogeneous.

Hall effect measurements are performed to calculate the density of charge carriers and the mobility of them in Pr_1-xSr_xNiO₃ before and after reduction to understand that process. The realization of that measure on our thin film on different substrates allow us to understand the real impact of the strain and the possible influence of defects generated during the growth or reduction process .

Keywords: Superconductivity, Nickelates, Reduction

References:

1.Li, D., Lee, K., Wang, B.Y. et al. Superconductivity in an infinite-layer nickelate. Nature 572, 624–627 (2019)

2.Motoki Osada, Bai Yang Wang, Kyuho Lee, Danfeng Li, and Harold Y. Hwang, Phase diagram of inifinite layer praseodymium nickelate Pr_1-xSr_xNiO₂ thin films, Phys. Rev. Materials 4, 121801(R) (2020)

3.Yaoyao Ji, Junhua Liu, Lin Li, Zhaoliang Liao, Superconductivity in infinite layer nickelates, J. Appl. Phys. 130, 060901 (2021)

Abstract

The great potential applications of metallophthalocyanines (MPcs) in fabricating organic ‎optoelectronic devices such as solar cells and light-emitting diodes in recent research are not ‎deniable. From this perspective, the study of the electronic structure of transparent metal oxide ‎‎(TMO) / MPc interfaces is counted as a focal point since these interfaces are determining the ‎charge carrier transport process (injection and extraction). So far, most reported studies have ‎been focused on the metal / MPc hybrid layers; therefore, the TMO / MPc interfaces and their ‎optical and electronic properties remain ambiguous enough to convince the researchers to work ‎on them. ‎

In this study, we have considered the TMO / MPc  interface creation and the variation of ‎the potential barrier between zinc oxide as a TMO electrode and the highest occupied ‎molecular orbital level of the hexadecafluoro cobalt phthalocyanine (CoPcF16) as the MPc ‎layer.  Mainly, the resulting interactions at the electrode / organic depend on the internal ‎‎(thickness, terminated atoms, roughness, etc.) and external (electric field, magnetic field, ‎thermal annealing, etc.) stimuli applied on TMO and MPCs. Here, the thickness of the CoPcF16, ‎hence the molecular rearrangement at the interface has been considered as the main factor. The ‎CoPcF16 was deposited in stepwise manner on the cleaned ZnO substrates while the thickness ‎of the CoPcF16 layer ranges from 0.5 angstrom to 7.0 nm. To thoroughly investigate the ‎interface creation from both: chemical and energy point of view, X-ray and ultraviolet ‎photoelectron spectroscopies (XPS and UPS) have been utilized. Finally, we demonstrate the ‎impact of the ZnO native defects on the TMO / MPc  interface creation.‎

Keywords: metal oxide, phthalocyanine, hybrid layer, valence band, CoPcF16, oxygen ‎vacancies.‎

CsPbBr₃ single crystal exhibits great potentials in the X-ray/gamma-ray spectroscopy and imaging. The understanding and suppressing of grown-in defects are expected to promote the CsPbBr₃ radiation detector performance. Here, an inverse temperature crystallization (ITC) method with modified precursor composition is proposed to prepare CsPbBr₃ single crystals. The introduction of adduct PbBr₂·2DMSO in the precursor solution gives rise to superior crystallization with lower impurity concentration and higher resistivity. The resulting CsPbBr₃ planar detectors achieve the high peak-to-valley ratio pulse height spectra with an energy resolution of 7.66%, illuminated by an uncollimated ²⁴¹Am @ 5.5 MeV alpha particle. Furthermore, we report the origin of liquid inclusion defects in solution-grown CsPbBr₃ crystals. Based on the finite element simulation, a model is developed combining concentration and temperature gradients, associated with the natural convection during crystallization, and is responsible for the liquid inclusions. By introducing forced convection to eliminate liquid inclusions, the crystalline quality of CsPbBr₃ has been significantly improved. The CsPbBr₃ crystal free from liquid inclusions enable the fabrication of an asymmetric planar Au/CsPbBr₃/Sn device with an energy resolution of 7.2 % illuminated under ¹³⁷Cs@662 keV γ-ray isotope.

References

[1] Wang, F.; Bai, R.; Sun, Q.; Liu, X.; Cheng, Y.; Xi, S.; Zhang, B.; Zhu, M.; Jiang, S.; Jie, W.; Xu, Y., Precursor engineering for solution method-grown spectroscopy-grade CsPbBr₃ crystals with high energy resolution. Chemistry of Materials 2022, 34 (9), 3993-4000.

[2] Bai, R.; Ge, B.; Liu, X.; Peng, X.; Zhang, X.; Liu, S.; Zhu, M.; Zhou, C.; Dubois, A.; Jie, W.; Xu, Y., Kinetic modulation-eliminated precursor liquid inclusions in solution-grown CsPbBr₃ bulk crystals for gamma-ray detection. Journal of Materials Chemistry A 2024.

Silicon carbide (SiC) is known as a host material for power electronics with extremely low loss. In addition, SiC is a good candidate for a host material for spin defects which can be applied to quantum bit and quantum sensor. Negatively charged silicon vacancy (V_Si) has spin = 3/2 and can act as spin defects for quantum technologies at room temperature. The spin for V_Si can be manipulated by optically detected magnetic resonance (ODMR) technique. In previous studies, magnetic field and temperature measurements using V_Si have been reported [1, 2]. Also, we demonstrated magnetic field sensing using V_Si up to 590 K. In addition, we developed the spin manipulation sequence to improve the sensitivity of temperature measurement for V_Si [4].

In this study, we create spin defects such as V_Si in SiC using energetic particle irradiation. Especially, we apply the particle beam writing (PBW) technique to create Vsi in certain locations of SiC devices. Temperature and magnetic field induced by electric current in SiC devices are measured using V_Si.

This study was partially supported by MEXT Q-LEAP (JPMXS118067395) and SIP 3rd “Promoting Application of Advanced Quantum Technologies to Social Challenges”.

[1] T. Ohshima et al., J. Phys. D: Appl. Phys. 51, 333002 (2018).

[2] T. M. Hoang et al., Appl. Phys. Lett. 118, 044001 (2021).

[3] S. Motoki et al., J. Appl. Phys. 133, 154402 (2023).

[4] Y. Yamazaki et al., Phys. Rev. Appl. 20, L031001 (2023).

Ion track-template has been widely considered as an unique method for the fabrication of one-dimensional nanostructures. For this purpose, understanding the fundamental process of track formation and fabricating novel nanostructures with unique geometrical and physical properties have been highly pursued in the past decades.

In this presentation, I will introduce the recent progress made on irradiation effects on polymers with swift heavy ions (SHI) and their applications in fabrication of novel nanostructures, with particular attention on their optical and mechanical properties. For the irradiation effects, we show that chemical structures of polyimide have been changed dramatically, resulting from SHI irradiation. Especially, some previously unidentified features have been disclosed, thanks to modified analysis methods. These findings may provide peculiar insight into fundamental mechanism of ion-polymer interaction and therefore potentially gives some hint for fabrication of functional nanostructures. Besides this, I will give my emphasis on the ion track-template fabrication of novel nanostructures and exploring their optical and mechanical properties. Typical nanostructures include elliptical nanowires, solid/hollow nanocones, quasi-BCC nanolattice mechanical metamaterials, and so on. Stemming from the unique geometrical features, their physical properties are discussed.

As the EUROfusion consortium communicates its transition from operating the science-driven ITER reactor to the industry-driven DEMO-class reactors, so evolve requirements for the liability, neutron radiation resistance, and accuracy of high-temperature magnetic diagnostics and plasma control systems. 

Łukasiewicz - IMiF has introduced a new brand to answer these challenges [1]. GET^®, or Graphene Epitaxy Technologies, offers an innovative graphene-based sensory platform for magnetic field detection [2-5]. The platform takes advantage of transfer-free p-type in-situ hydrogen-intercalated quasi-free-standing graphene epitaxially grown on semi-insulating on-axis SiC using the CVD method [6-9]. It is protected against environmental conditions by amorphous atomic-layer-deposited aluminum oxide passivation, synthesized from trimethylaluminum and deionized water at 770 K [10-11].

The sensors come in two variants. The one on vanadium-compensated on-axis 6H-SiC(0001) offers current-mode sensitivity of 140 V/AT up to 573 K, the other on high-purity on-axis 4H-SiC(0001) offers 80 V/AT but up to 770 K. The sensitivity and thermal range of operation are already superior to the traditional technology of thin-film metallic layers based on Bi, Cr, Au, Mo, Ta, or Cu. 

The 4H-SiC device performance is further boosted by pre-epitaxial ion implantation that reconstructs the SiC defect structure and eliminates deep electron traps related to silicon vacancies. The modification suppresses the thermal build-up of a detrimental electron channel and improves the stability, linearity, and reproducibility of the sensor sensitivity curves [12].

The platform is greatly resistant to fast-neutron radiation of a fluence of 6.6 × 10¹⁷ cm^-2 and possesses a self-healing property [13]. Further to this, the self-healing effect exhibits a fluence and thermal threshold, suggesting that the demanding conditions of the tokamak may prove in favor of the platform.  

[1] www.graphene2get.com.
[2] T. Ciuk, et al., Appl. Phys. Lett. 108, 223504, (2016).
[3] T. Ciuk, et al., Carbon 139, 776–781 (2018).
[4] T. Ciuk, et al., IEEE Trans. Electron Dev. 66, 3134–3138 (2019).
[5] A. Dobrowolski, J. Jagiello, D. Czolak, T. Ciuk, Phys. E: Low-Dimens. Syst. Nanostructures 134, 114853 (2021).
[6] T. Ciuk, et al., J. Appl. Phys. 116, 123708 (2014).
[7] T. Ciuk, W. Strupinski, Carbon 93, 1042–1049 (2015).
[8] T. Ciuk, P. Caban, W. Strupinski, Carbon 101, 431–438 (2016).
[9] A. Dobrowolski, J. Jagiello, K. Pietak-Jurczak, M. Wzorek, D. Czolak, T. Ciuk, Appl. Surf. Sci. 642, 158617 (2024).
[10] K. Pietak, J. Jagie l lo, A. Dobrowolski, R. Budzich, A. Wysmo lek, and T. Ciuk, Appl. Phys. Lett. 120, 063105 (2022).
[11] K. Pietak-Jurczak, J. Gaca, A. Dobrowolski, J. Jagie l lo, M. Wzorek, A. Zalewska, T. Ciuk, , ACS Appl. Electron. Mater. 6, 1729–1739 (2024).
[12] T. Ciuk, et al., Carbon Trends 13, 100303 (2023).
[13] S. El-Ahmar, M. J. Szary, T. Ciuk, et al., Appl. Surf. Sci. 590, 152992 (2022).

The industrial progress of the 21st century could greatly benefit from developing and exploiting fusion reactors producing environmentally clean friendly electrical energy. A key problem here is the need for new advanced materials able to operate under extreme conditions (high temperatures and intensive neutron/gamma radiation).

In this report, we will provide a brief overview of both general information on the status of the problems and the most interesting results obtained within the two EUROfusion Enabling Research Projects – “Advanced experimental and theoretical analysis of defect evolution and structural disordering in optical and dielectric materials for fusion applications (AETA)” (2019-2020) and “Investigation of defects and disorder in nonirradiated and irradiated Doped Diamond and Related Materials for fusion diagnostic applications (DDRM) – Theoretical and Experimental analysis“ (2021-2023).

In a series of joint works by ISSP UL (Latvia), UT (Estonia) and KIT (Germany), radiation damage of some promising functional materials (Al2O3, MgAl2O4, SiO2, diamond and a few more) from the priority list of the EUROfusion consortium was studied under neutron, proton, heavy ion.

The optical and dielectric, vibrational and magnetic properties of numerous crystalline and ceramic materials were carefully studied. Based on this study, we developed new theoretical methods able to evaluate and predict some important properties of these materials as well as their radiation damage evolution under extreme reactor conditions.

We will also demonstrate with examples what experimental capabilities are available at the institute for the successful analysis of radiation defects and related processes, which can be useful for initiating new projects.

Tuning single-photon emission via controlling the H-induced defect complex in dilute III-V Nitride nanowires

A. Sagar Sharma¹ P. De Vincenzi¹, N. Denis², F. Santangeli¹, R. Pallucchi¹, S. Cianci¹, A. Polimeni¹, M. Yukimune³, F. Ishikawa³, M. De Luca¹

¹Physics Department, University of Sapienza, Rome, Italy

²Department Physik, Universität Basel, Klingelbergstrasse 82, Basel, Switzerland

³Research Center for Integrated Quantum Electronics, Hokkaido University, Sapporo, Japan

Abstract

Developing a finely energy-tunable solid-state single-photon source is a major challenge in the field of quantum communication and computation, and bandgap engineering at the nanoscale is a very promising, though challenging, solution to this issue. In this work, we have used GaAs/GaAsN core/multi-shell nanowires (NWs), with N content 0.7 to 3 at% [1]. N is the smallest among the other group V elements and has higher electronegativity, hence the incorporation of such a low concentration of N in GaAs creates a strong localized field in the host GaAs that results in a large and counterintuitive bandgap reduction of up to ≈480 meV for N=3%. By using micro-photoluminescence (PL) spectroscopy,  we demonstrate how the post-growth irradiation of H-ions tunes the bandgap of single GaAs/GaAsN NWs on a large energy scale. Treatment with H-ion creates indeed an N-H defect complex which eliminates the perturbations induced by N incorporation in the GaAs lattice, therefore we can practically tune the bandgap of GaAsN up to the value of GaAs [2]. Through the subsequent thermal annealing of the wires, we have liberated the H from the N-H complex defect and partially or completely (depending on the desired bandgap energy) restored the effects of N incorporation in the GaAs lattice. This proves the positive degree of reversibility of this approach. By exploiting this, low temperature (5K) PL measurements of partially hydrogenated wires show single photon emission behaviour, assessed by autocorrelation measurements (g(2)(0)). The source of single photon emission is identified as clusters of a few N atoms in the GaAs lattice. This process can be engineered by irradiating H-ions to achieve the tunability of single photon emission. The overall results suggest a promising route for creating an energy-controlled single photon emitter, fully integrated with a Si photonic chip.   

References:

[1] M Yukimune et al 2019 Nanotech. 30 244002

[2] F Biccari et al 2018 Adv. Mater. 30, 1705450

Strain can significantly modulate structural, optical and electronic properties of semiconductors [1]. The corresponding mechanism is usually regarded in terms of the twist, extension or contraction of chemical bonding causing the crystal lattice deformation. In case of ZnO, effect of strain is doubly interesting. Firstly, because this transparent semiconductor can be applied in flexible electronics, secondly, because strain in ZnO influences formation energy of defect states, and thus their abundance. As has been recently established, high dispersion of ZnO conductivity, reaching 4 orders of magnitude for undoped material, is caused by the presence of defect complexes which include point defects, hydrogen and, in the case of a doped material, also a dopant. These complexes are origin of shallow donor and acceptor states that strongly change the conductivity of the material. On the other hand, defect complexes cause crystal lattice distortion, so it might be assumed that strain and microstrain have a significant impact on the formation of defect complexes and thus on the conductivity of this material [2].

In the present paper we present Density Functional Theory calculations showing that formation energy of the NOVZn and NOVZnHx complexes, that provide shallow and deep acceptor levels in N-doped ZnO, strongly depend on strain. In particular, Nudged  Elastic  Band calculations show  that, as example,  4 % compressive axial strain  along z-axis reduces the migration barrier, E_a of zinc vacancy moving around the NH₂ of about 0.25 eV, and strain  in the xy-plane  leads to the lowering  of E_a  to  zero, that might cause the grouping of acceptors in crystallites with different strain.  We present the experimental results that confirm this hypothesis. The experimental study was performed on ZnO:N films grown by Atomic Layer Deposition. The films were deposited in the same growth process on two differently oriented sapphire substrates (c-Al₂O₃ and a-Al₂O₃). As previously shown, although the growth is not epitaxial, the crystallographic structure strongly depends on substrate orientation [3]. Low-temperature photoluminescence spectra reveal that the intensity of acceptor-related emission is much stronger for ZnO/a-Al₂O₃ films as compared to ZnO/c-Al₂O₃ ones, which give evidence for influence of stress on the formation of acceptor complexes in zinc oxide. This finding is confirmed by cathodoluminescence images taken on the ZnO:N/Si films cross-sections showing the acceptor and donor domains situated in different crystallites in polycrystalline ZnO:N films [4].

The study was partially supported by the Polish NCN Project DEC-2018/07/B/ST3/03576

References

[1] I. Mosquera-Lois, S.R. Kawanagh, A. Walsh and D.O. Scanlon, npj Comp. Mat. 9, 25 (2023)

[2] O. Vonianska, V.Yu. Ivanov, L. Wachnicki, E. Guziewicz, ACS Omega (2023)

[3] S. Mishra et al., Materials 16, 151 (2023)

[4] S. Mishra, B.S. Witkowski et al., Phys. Stat. Sol. A 2022, 2200466 (1-11)

Photoluminescence in SrTiO3 through Strain Engineering

Eric Brand1, Daesung Park1*, Nini Pryds1*

Department of Energy Conversion and Storage, Technical University of Denmark (DTU), Building 310, 2800 Kgs Lyngby, Denmark.

E-mail: daepa@dtu.dk, nipr@dtu.dk

Color centers are microscopic defects in a crystal lattice, often intentionally created by introducing electrons into point defects, e.g., vacancies, and they become optically active when they exhibit optical transitions between their excited and ground states. Such point defects in host materials are essential components for next generation quantum sensing and information applications such as optical communication, ultrafast processes in semiconductors, and high-resolution molecular spectroscopy. For example, the optical properties of ABO3 perovskite oxides can be significantly modified by incorporating active impurities, e.g., a Cr3+ substitute at the Ti site of SrTiO3 (STO). The Cr3+ photoluminescence (PL) emission in bulk STO is well-known and is characterized by a sharp zero phonon line (ZPL) at ~790 nm [1]. Moreover, the Cr3+ ZPL emission in bulk STO strongly depends on the electron-phonon coupling, showing a prominent emission at the tetragonal-cubic phase transition temperature (approximately 105 - 120 K). Further, tuning the lattice strain of STO is an effective way to modify the photoluminescence properties [2]. In this work, we demonstrate tuning the Cr3+ emission in STO single crystals by varying the doses of Cr ion implantation. Our results show that the Cr implantation induces out-of-plane strain in the STO, resulting in a tetragonal distortion of the host crystal structure (up to 3.5%). The induced tetragonality in STO leads to a temperature-independent Cr3+ emission, in contrast to a strong temperature-dependent emission feature of conventional STO. To understand the optical transition in STO, we carried out temperature-dependent PL and X-ray diffraction measurements. The results offer new perspectives for understanding the transition in STO and the possibility of engineering color centers in oxides towards quantum sensing at/near room temperature.

References:

[1] Sihvonen, Y. T. "Photoluminescence, photocurrent, and phase‐transition correlations in SrTiO3." Journal of Applied Physics 38.11 (1967): 4431-4435.

[2] Dong, Zhengang, et al. "Enhanced upconversion photoluminescence assisted by flexoelectric field in oxide nanomembranes." Laser & Photonics Reviews 16.4 (2022): 2100454.

Two-dimensional transition metal dichalcogenides (TMDCs) have emerged as promising candidates for next-generation nanoelectronics, owing to their distinctive crystal structures and an abundance of intrinsic defects. In our study, we have undertaken a series of investigations centered on the defect engineering of TMDCs monolayers and TMDCs based heterostructures. For monolayer TMDCs, defective structures can enhance the electronic density of states and out-of-plane electronic transfer, augmenting specific optoelectronic functionalities. For instance, we have successfully elevated the performance of WSe2 as a substrate for surface-enhanced Raman scattering (SERS) by adjusting its atomic ratio. Experimental findings revealed that when the atomic ratio of WSe2 was adjusted to 1.96, its maximum enhancement factor (EF) could surpass 120, representing a more than 40-fold increase compared to pristine WSe2. Based on two-dimensional heterostructures, we have developed a methodology for the automated construction of lateral diodes, exemplified by graphene and MoSe2 (MoSe2/G). By utilizing focused ion beam writing technology to introduce selenium defects, unique electronic properties were imparted to the heterostructure, enabling the realization of rectification and current regulation functions in two-dimensional diodes. Leveraging this two-dimensional diode architecture, we have fabricated a high-performance self-powered photodetector, characterized by a broadband photoresponse (spanning from 450 nm to 1064 nm), a high on/off ratio (104), rapid response times (ranging from 20 µs to 60 µs), and high responsivity (0.74 A/W). Besides, we have further developed a controllable oxygen-doped SnS2/G heterostructure diode-type gas sensor based on the two-dimensional photodetector architecture, which possessed high selectivity for nitrogen dioxide and exhibited swift response/recovery characteristics. Our study has effectively modulated the optoelectronic properties of 2D TMDCs via defect engineering, creating diverse multifunctional heterostructures and devices, thus unlocking the immense potential of defect engineering in optimizing TMDC performance and expanding their application spectrum.

Transition metal dichalcogenide monolayers are two dimensional (2D) direct bandgap semiconductors with improved light-matter interactions and carrier mobility, this makes them an excellent component in optoelectronic devices [1]. But the atomically thin nature of these materials results in a weak (<1%) light absorption and low photocurrents.  For instance, monolayer MoS2, with a bandgap of 1.8 eV, can only detect light with energy above approximately 680 nm, limiting its use in broadband applications [2]. Poor response times, and bandgap energy limitations in case of MoS2 makes it difficult to realize optoelectronic devices [2]. Integration with strong light absorbing nanomaterial, such as zero dimensional (0D) nanocrystals (InAs NCs) can enhance their interaction with light and improve optoelectronic device performance [3,4]. In this contribution, we report an efficient charge transfer in a 0D-2D heterostructure of InAs/ZnSe core shell nanocrystal and MoS2 monolayer, forming a type II heterostructure (with InAs/ZnSe being the lower bandgap material). We use steady-state, time-resolved µ-PL spectroscopy, and photocurrent measurement techniques to probe the photo-induced charge transfer. Upon excitation of both materials efficient hole transfer to InAs is observed, with selective excitation of InAs, electron transfer from InAs to MoS2 monolayer is observed. The heterostructure exhibits a broadband photoresponse from 300 to 850 nm with a responsivity of 103 A/W. The signal-to-noise ratio substantially increases by 3 to 4 orders of magnitude for 700 and 850 nm excitation compared to pristine MoS2. A photodetector device of this hybrid structure drastically improves both the detection range and response times.

 References

[1]   A. Splendiani, et al.,  Nano Lett 2010, 10, 1271. 

[2]   O. Lopez-Sanchez, et al., Nat Nanotechnol 2013, 8, 497. 

[3] Z. Liu et al., J. Am. Chem. Soc. 2023, 145, 33, 18329–18339

[4]   R. A. Yotter, et a., IEEE Sens J 2003, 3, 288. 

Biofouling begins with the accumulation of microorganisms (microfouling) on surfaces exposed to humid and marine environments, causing performance issues and economic losses. Traditional anti-biofouling methods include surface modifications, bactericidal materials, and enzyme-based treatments. Nanozymes, with haloperoxidase (hPOD) activity, express bactericidal activity and offer promising eco-friendly solutions to biofouling. This study focuses on defect-engineered bismuth telluride (Bi₂Te₃) nanosheets, through controlled NaOH (X) etching, creating Bi³⁺-rich defective Bi₂Te₃ (d-Bi₂Te₃-X) with enhanced hPOD activity. The Bi³⁺ species reacts with hydrogen peroxide (H₂O₂) and bromide ions (Br⁻) to produce antibacterial hypobromous acid (HO-Br) and singlet oxygen (¹O₂). The optimal d-Bi₂Te₃-250 nanosheets exhibit an 8-fold increase in hPOD activity compared to as-grown Bi₂Te₃. Bacterial viability assays revealed significant antibacterial effects, with only 1% viability for Staphylococcus aureus and 45% for Pseudomonas aeruginosa. Additionally, the d-Bi₂Te₃-250 nanozymes effectively inhibited Pseudomonas aeruginosa biofilms, demonstrating up to 73% reduction in biofouling. This research highlights that defect engineering can significantly enhance the hPOD activity of Bi₂Te₃ nanosheets, offering a cost-effective and efficient alternative to expensive noble metals and transition metal chalcogenides for anti-microfouling.

Two-dimensional materials provide a unique platform for investigating thermal transport at the nanoscale. Recently, there has been significant interest in amorphous 2D materials, such as amorphous boron nitride or amorphous carbon, due to their ultralow dielectric constant and ability to enhance the carrier mobility of 2D materials. As these materials approach integration into real devices, it is essential to characterize their thermal properties.

From a thermal perspective, amorphous materials exhibit extremely low thermal conductivity (k) well below 1 W/mK due to the lack of a crystalline structure, functioning effectively as heat-insulating layers. In this study, we present the thermal characterization of aBN prepared by atomic layer deposition using borazine precursor, as a function of thickness and element concentration. The cross-plane thermal conductivity of the aBN films, ranging from few nms to bulk, were investigated using frequency domain thermoreflectance. The thermal properties were be modeled through classical molecular dynamics (MD), with EMD calculations exploring thermal conductivity and spatial features of lattice vibrations (localized vs extended modes).

However, achieving the theoretical amorphous limit of thermal conductivity is not exclusive to amorphous materials and can also be realized in crystalline materials through well-designed nanofabrication. To this end, we propose MoS₂ phononic crystals (PnCs) designed to effectively reduce k while minimally impacting electrical conductivity, providing a potential solution for thermal management in nano-electronics. In suspended MoS₂ PnCs, an exceptionally low k down to 0.1 W/mK, below the amorphous limit. Molecular dynamic simulations, accounting for film thickness, porosity, and temperature, support these findings. The efficacy of this approach is demonstrated by fabricating suspended heat-routing structures that effectively confine and guide heat flow in pre-specified directions. Therefore, controlling the defects in 2D materials is a useful approach for controlling their thermal conductivities.