Impurity Doping Research Articles

Effect of realistic out-of-plane dopant potentials on the superfluid density of overdoped cuprates

Recent experimental papers on hole-doped overdoped cuprates have argued that a series of observations showing unexpected behavior in the superconducting state imply the breakdown of the quasiparticle-based Landau-BCS paradigm in that doping range. In contrast, some of the present authors have argued that a phenomenological "dirty $d$-wave" theoretical analysis explains essentially all aspects of thermodynamic and transport properties in the superconducting state, provided the unusual effects of weak, out-of-plane dopant impurities are properly accounted for. Here we attempt to place this theory on a more quantitative basis by performing $\textit{ab-initio}$ calculations of dopant impurity potentials for LSCO and Tl-2201. These potentials are more complex than the pointlike impurity models considered previously, and require calculation of forward scattering corrections to transport properties. Including realistic, ARPES-derived bandstructures, Fermi liquid renormalizations, and vertex corrections, we show that the theory can explain semiquantitatively the unusual superfluid density measurements of the two most studied overdoped materials.

Tunable interface states between Floquet-Weyl semimetals

Weyl semimetals and nodal line semimetals are characterized by linear electronic bands touching at zero-dimensional points and one-dimensional lines, respectively. Recently, it has been predicted that nodal line semimetals can be driven into tunable Floquet-Weyl semimetals by circularly polarized light. Here, we study the occurrence of interface states between two regions of a nodal line semimetal shined by two beams of light with opposite circular polarizations. Within a minimal model, we find remarkable modifications of the energy structure by tuning the polarized light, such as the possible generation of van Hove singularities. Moreover, by adding a $\delta$-doping of magnetic impurities along the interfacial plane, we show the occurrence of a switchable and topologically non-trivial, vortex-like pseudo-spin pattern of the interface states.

Thermal conductivity in fluorine-doped single-walled carbon nanotubes

Herein, we report on the axial thermal conductivity (κzz) of a non-degenerate fluorine-doped single-walled carbon nanotubes (FSWCNTs) under single and dual relaxation models. We derived the expressions for the carrier thermal current density (Q→), carrier thermal conductivity (κe), and simulated the lattice thermal conductivity (κℓ) using Callaway’s dual relaxation time model as a function of temperature field. The thermal conductivity was observed to be strongly nonlinear and purely phonon dominated. Moreover, the study investigated the effect of impurity concentration (no), and overlapping integrals for jumps (Δs and Δz) on κzz. Our results showed that, the axial thermal conductivity (κzz) and the operational temperature range of FSWCNTs could be tuned using doping (impurity) and overlapping integrals. The ac component of the electric field was observed to strongly energise the FSWCNT carrier gas and thus, caused the carrier contribution to the axial thermal conductivity to be non-significant but solely phonon dominated. Interestingly, using the Callaway’s model, κℓ was evaluated to be 107.201 W/mK, at 300 K. Thus, base on the FSWCNT’s low κℓ and its possibility of tuning to operate at high temperatures makes it a good choice for thermoelectric applications including chip level cooling, heat sinks and power generation.

Modeling of temperature dependence of Λ-graded InGaN solar cells for both strained and relaxed features

The temperature dependence of Λ-graded InGaN solar cells is studied through simulation using nextnano software. Λ-Graded structures have been designed by increasing and then decreasing the indium composition in epitaxial InGaN layers. Due to polarization doping, layers of p-type and n-type doping arise without the need for impurity doping. Different individual structures are designed by varying the indium alloy profile from GaN to maximum indium concentrations, xmax, ranging from 20% to 90% pseudomorphically strained to a GaN substrate and from 20% to 100% for completely strain-relaxed materials and linearly decreases back to GaN. For InxGa1-xN with x>∼0.9, if the material is strained to the GaN lattice constant, it is predicted to have a negative band gap. So this case is not considered here. The temperature dependence of the electrical and optical properties as they relate to the solar efficiency of the Λ-graded structures under relaxed and strained conditions are studied. Additionally, the dimensionless absorption coefficients are fitted and plotted as functions of the band gap under both strained and relaxed conditions at different temperatures. As a result, the generation rates as functions of the penetration depth within a cell can be calculated in order to obtain the solar cell parameters including efficiency for each Λ-graded structure at different temperatures. Under the strained condition, the xmax, where the solar cell efficiency reaches its maximum for each temperature, decreases as the temperature increases. At the same time, under the relaxed condition, at low temperatures (T = 100–400 K), xmax is 100%, that is, grading to InN results in maximum efficiency, while at higher temperatures (T = 500–800 K), xmax decreases with the increasing temperature.

Local structures of phosphorus atoms implanted in crystalline diamond

Effective impurity doping into diamond by an ion implantation technique has been one of the crucial issues for realizing diamond-based high-power electronic devices. Especially for n-type impurity doping, the electrical activation has not been accomplished yet in a practically available level. In this study, local structures and depth profiles of implanted phosphorus atoms were studied by x-ray absorption spectroscopy, secondary ion mass spectroscopy, and first-principles calculations. P ion implantations were performed at two extreme substrate temperatures of room temperature and 900°C at multiple incident energies from 10 to 150 keV for flat doping and a single energy of 200 keV for δ-doping followed by activation annealing at 1300°C. The x-ray absorption spectra and the theoretical calculation showed that most of the implanted phosphorus atoms implanted with a flat doping concentration are existent in the substitutional site; however, they seem to bond with hydroxyl or vacancy complexes, probably resulting in electrical inactivation. Indeed, secondary ion mass analysis showed that a large number of O and H atoms are distributed in the P-doped layer, probably diffused from the surface through a damaged network. On the other hand, impurity diffusion was not observed in the P δ-doped sample followed by high-temperature annealing with a cap layer. It is clearly suggested that a damaged layer by ion irradiation near the surface acts as diffusion channels and trap sites of various impurities. High-temperature annealing with a cap layer is also quite effective in suppressing the formation of the defective layer that becomes a diffusion path of O and H.

Investigating the influence of Ni doping on the CuO thin films deposited via ultrasonic spray pyrolysis: Structural, optical and H2 gas sensing analyses

Pure and nickel doped copper oxide thin films were successfully grown on a soda lime glass substrates including various amounts (1, 2, 4, and 8%) of nickel dopants via ultrasonic spray pyrolysis method. The effects of the different impurity doping concentrations on the structural, morphological, optical, and H2 gas sensing properties of the prepared samples were assessed by performing X-ray Diffraction, scanning electron microscopy, ultraviolet–visible spectroscopy, photoluminescence spectroscopy, gas sensing measurements, and X-ray photoelectron spectroscopy. The structural observations revealed that the grown films have polycrystalline nature with monoclinic cubic crystal structure of copper oxide with no secondary phases present, and grain sizes increased in general as the doping percentage increased except for the 4% nickel doped sample. The morphological analysis showed that the impurities clearly led to modifications in the shapes of the grown copper oxide nanocrystals. In the optical measurements, it was uncovered that the ideal impurity level can enhance the absorbance features of the copper oxide material, and a slight increase in bandgap occurred due to the first level of impurity doping, but then the bandgap decreased as the impurity contents increased. A gradual decrease in the intensity of luminescent emissions was observed in the photoluminescence spectra as the doping concentration increased, which may be associated with the defects caused by the added nickel ions into the copper oxide lattices. Gas sensor measurements disclosed that the produced films were responsive when exposed to H2 gas, and enhanced responsivity was realized by the nickel doping compared to that of pure copper oxide sample. X-ray photoelectron spectroscopy confirmed the presence of aimed chemical compositions in the prepared thin films.

Slow Hole Localization and Fast Electron Cooling in Cu-Doped InP/ZnSe Quantum Dots.

Impurity doping of low-dimensional semiconductors is an interesting route towards achieving control over carrier dynamics and energetics, e.g., to improve hot carrier extraction, or to obtain strongly Stokes shifted luminescence. Such studies remain, however, underexplored for the emerging family of III-V colloidal quantum dots (QDs). Here, we show through a detailed global analysis of multiresonant pump-probe spectroscopy that electron cooling in copper-doped InP quantum dot (QDs) proceeds on subpicosecond time scales. Conversely, hole localization on Cu dopants is remarkably slow (1.8 ps), yet still leads to very efficient subgap emission. Due to this slow hole localization, common Auger assisted pathways in electron cooling cannot be blocked by Cu doping III-V systems, in contrast with the case of II-VI QDs. Finally, we argue that the structural relaxation around the Cu dopants, estimated to impart a reorganization energy of 220 meV, most likely proceeds simultaneously with the localization itself leading to efficient luminescence.

Optimization of heterostructure design for switching pHEMT transistors

In this work, the design of one-side-doped pHEMT heterostructures was varied to obtain better microwave switching transistor parameters. For this purpose, the doping level, the method of introducing the doping impurity and the type of spacer were varied. It was obtained that the electron density in the heterostructures increases without saturation up to a sheet donor concentration of 9 × 1013 cm−2. In the sheet donor concentration range of (5–6) × 1012 cm−2 a sharp increase in electron density in δ-doped heterostructures was observed. At an optimum concentration of the donors of 6 × 1012 cm−2 the conductivity of heterostructures with modulation doping from the supply layer is 9% higher than that of δ-doped heterostructures. The use of a spacer in the form of an AlAs/GaAs superlattice leads to a decrease in the electron density, which is more pronounced for δ-doped heterostructures. But the conductivity of heterostructures with AlAs/GaAs spacer is higher due to better mobility. Measurements of electron density and mobility, as well as the conductivity of heterostructures together with the calculation of electron concentration at different gate biases by the self-consistent solution of Schrödinger and Poisson equations showed that δ-doped heterostructures at 6 × 1012 cm−2 and AlAs/GaAs spacers are most preferable for switching transistors. These heterostructures were used to fabricate monolithic integrated circuits of single-pole double throw pHEMT-based switches with gate length and widths of 0.5 μm and 100 μm, respectively. The resulting transistors had the following parameters: gm = 400 mS/mm, saturation current density ID = 380 mA/mm, ON-state resistance 1.0 Ohm × mm, OFF-state capacitance 0.37 pF/mm. Parameters of the fabricated switches at frequency of 20 GHz are the following: insertion loss 2.2 dB, isolation 56 dB, return loss 11.7 dB, linearity P1dB 21 dBm and IIP3 - 40 dBm.

(Digital Presentation) Characterization of Light Emission Properties of Impurity Doped Ge/Si Core–Shell Quantum Dots

Si–based quantum dots (QDs) have attracted much attention as an active element in Si-based optoelectronic applications because their light emission properties due to carrier confinement have potential to combine photonic processing with electronic processing on a single chip. We have focused on CVD formation and characterization of Si-QDs with Ge core and reported their photoluminescence (PL) properties attributable to type II energy-band alignment between the Ge–core and the Si-shell [1-2]. In addition, we have also demonstrated stable electroluminescence in the near–infrared region from diode structures having a 3–fold stacked Si–QDs with Ge core with an areal dot density of ~2.0×1011 cm−2 under pulsed bias applications [3]. One of the possible ways to enhance the radiative recombination rate and the improve the light emission efficiency is to reduce non–radiative centers and to increase carrier density by impurity doping into the QDs. In this work, in extending our research work to valency-controlled Si–QDs with Ge core, we have studied the effects of boron– and phosphorus–doping to the Ge core on PL properties.A highly dense layer of Si–QDs with Ge core was formed by controlling thermal decomposition of GeH4 and SiH4 alternately on thermally–grown ~2.0 nm–thick SiO2. During the Ge deposition, delta doping of boron and phosphorus atoms in QDs was carried out by pulse injection using 1% B2H6 and 1% PH3 diluted with He, respectively. No changes in dot size and areal dot density with B or P doping were confirmed by AFM topographic images. Under photoexcitation of undoped QDs with a 979–nm line from a semiconductor laser, a broad PL spectrum, which consists of four Gaussian components originating from radiative recombination through quantized states in QDs, were observed in the energy range from 0.62 to 0.85 eV even at room temperature. For the doped QDs, in addition to the four components seen in undoped QDs, relatively-narrow components peaked at ~0.64 eV and ~0.68eV were observed with B-doping and with P-doping to the Ge core, respectively. Notice that, with an increase in B2H6 pulse injection from 1 to 4 times, the integrated PL intensity was enhanced by a factor of 1.4 to 2.4 compared to that of the undoped QDs while no significant change in spectral shape was observable. This can be interpreted in terms of an increase in the number of holes with B-doping to the Ge core since the carrier recombination rates is proportional to the product of the number of electrons and holes confined in each of QDs under weak photoexcitation.AcknowledgementsThe authors wish to thank Dr. Akio Ohta for his contribution to the measurements. This work was supported in part by Grant–in–Aid for Scientific Research (A) 19H00762, 21H04559, and Fund for the Promotion of Joint Intentional Research [Fostering Joint International Research (A)] 18KK0409 of MEXT Japan.

(Invited, Digital Presentation) Physical Properties of Silicon-Germanium Nanostructures: Theory and Simulations

Silicon germanium nanostructures (SiGe NSs) have acquired today a prominent role in several cutting-edge research topics in nanoscience, thanks to the most recent advances in synthesis, processing and characterization [1-4]. The physical properties of such nanosystems are strictly related not solely to the size of the system (as in pure Si and Ge NSs), but also to the relative composition of Si and Ge atoms as well as to the geometry of Si/Ge interface. The investigation of these materials with experimental techniques is, however, complicated by several factors not always well controlled which can hide the right comprehension of the fundamental properties. In this context first principles theoretical modeling is extraordinarily important as it can complement or augment experimental observations.Here, I will present an overview of the ab initio computational modeling of various types of SiGe nanosystems (nanowires [5], nanodots [6] and slabs [7-8]). I will outline how by bringing together two similar elements –Si and Ge, neighbors in the periodic table–, a broad variety of new chemical and physical properties emerge, stimulating both fundamental and application-driven research in nanoscience. Indeed, I will show that substituting some of the atoms of a pure Si NS with Ge in configurations of distinct compositions and dimensionalities, can strongly affect some fundamental properties such as band gap, band offsets, work function and impurity doping levels. Bibliography [1] M. Amato, M. Palummo, R. Rurali, S. Ossicini, Chem. Rev. 114, 1371 (2014)[2] I. Berbezier, A. Ronda, Surf. Sci. Rep., 64, 47 (2009)[3] J.-N Aqua, I. Berbezier, L. Favre, T. Frisch, A. Ronda, Phys. Rep., 522, 52 (2013)[4] D. J. Paul, Semicond. Sci. Technol. 19, R75–R108 (2004)[5] M. Amato, S. Ossicini and R. Rurali, Nano Lett. 11, 2, 594-598 (2011)[6] I. Marri, M. Amato, S. Ossicini, S. Grillo, O. Pulci, Interplay of quantum confinement and strain effects in type I to type II transition in Ge/Si core-shell nanocrystals (submitted, 2022)[7] I. Marri, M. Amato, M. Bertocchi, A. Ferretti, D. Varsano, S. Ossicini, Phys. Chem. Chem. Phys. 22, 25593-25605 (2020)[8] S. Pouch, M. Amato, M. Bertocchi, S. Ossicini, N. Chevalier, T. Mélin, J.M. Hartmann, O. Renault, V. Delaye, D. Mariolle, L. Borowik, J. Phys. Chem. C 119, 26776-26782 (2015)

Electrical and optical properties and defects of (100)- and (001)-oriented V-doped β-Ga2O3 crystals grown by EFG

Altering the n-type conductivity and optical properties of ultra-wide bandgap β-Ga2O3 by impurity doping has been a topic of research interest in the semiconductor field. Simultaneously, β-Ga2O3 with a monoclinic structure has exhibited interesting anisotropy. In this study, (100) and (001) V-doped β-Ga2O3 crystals were synthesized by the edge-defined film-fed growth (EFG) method, and their structural, electrical and optical properties, and defects were systematically investigated. V-doped β-Ga2O3 crystals with a (100) plane have a better crystalline quality and flatter surface than those with a (001) plane. Compared with undoped crystals (∼1016 cm−3), the carrier concentration of the (100) and (001) V-doped β-Ga2O3 substrates only increased to 4.30 × 1017 cm−3 and 1.92 × 1017 cm−3, respectively, which was related to the VxOy clusters formed by excess V as trapping traps and the carbon DX center as an electron capture trap formed by the relaxation of C atoms in the direction opposite to that of the C–O bond. The generation of absorption peaks and flat regions in the transmittance spectra was attributed to the introduction of V doping. The etch pit morphology and Raman peak intensity on the surfaces of the (100) and (001) V-doped β-Ga2O3 substrates were different owing to the anisotropy of the monoclinic lattice structure. These results contribute to the understanding of the influence of V and anisotropy on β-Ga2O3 crystals and expand their applications in electrical and optical fields.

Thermal conductivity and phonon anharmonicity of chemical vapor transport grown and mineral–FeS2single crystals: An optothermal Raman study

AbstractIn this work, thermal conductivity and anharmonic properties of chemical vapor transport (CVT)‐grown pyrite‐FeS2and mineral single crystals have been investigated and compared. It has been shown that optothermal Raman technique is able to capture the large thermal conductivity difference between the CVT and mineral samples at low temperatures. This difference is attributed to point defects such as sulfur vacancies or impurity doping of CVT‐grown FeS2crystals. Balkanski–Klemens model analysis of the samples has shown that three‐phonon scattering of Agand Egmodes and the lattice thermal expansion are the dominant anharmonic contributors while four‐phonon scattering is negligible in pyrite‐FeS2. Thus, thermal conductivity of materials that is difficult to measure by conventional methods (i.e., flash method) can be measured in their most native form by using optothermal Raman spectroscopy (RS) without rigorous sample preparation.

Density changes in amorphous silicon induced by swift heavy ions

Pure and gold-doped amorphous silicon membranes were irradiated with swift heavy ions (75 MeV Ag or 1.1 GeV Au ions) and studied using small-angle x-ray scattering. The samples that were irradiated with 1.1 GeV Au ions produced a scattering pattern consistent with core-shell-type ion tracks of 2.0 $\ifmmode\pm\else\textpm\fi{}$ 0.1 nm (core) and 7.0 $\ifmmode\pm\else\textpm\fi{}$ 0.3 nm (total) radius irrespective of gold doping, consistent with radii previously observed [9]. The density in the core is nearly 4% different from that of the surrounding material. The entire track (core $+$ shell) is slightly less dense than the surrounding material, yielding an expansion or hammering constant $A$ of $0.036\ifmmode\pm\else\textpm\fi{}0.003$ ${\mathrm{nm}}^{2}$ per ion track, consistent with the macroscopic ``hammering'' deformation. No tracks were found in samples irradiated with 75 MeV Ag ions, and no signature specific to the gold impurity doping could be observed.

Characterization of Light Emission Properties of Impurity Doped Ge/Si Core–Shell Quantum Dots

We have fabricated valency-controlled Si-QDs with Ge-core with an areal density as high as ~1011 cm-2 on ultrathin SiO2 and studied the effect of phosphorus- and boron-doping on Ge-core from their PL properties. During the Ge deposition, delta doping of phosphorus or boron atoms in QDs was carried out by pulse injection using 1% PH3 or B2H6 diluted with He, respectively. No changes in dot size and density with either P- or B-doping were confirmed by AFM topographic images. Under photoexcitation of undoped QDs with a 976-nm line from a semiconductor laser, broad PL spectra consisting of four Gaussian components originating from radiative recombination through quantized states in QDs were observed in the energy range from 0.62 to 0.85 eV without impurity doping even at room temperature. In the doped QDs, relatively-narrow components peaked at ~0.68 eV and ~0.64eV were observed with P-doping and with B-doping to Ge-core, respectively, in addition to the four components seen in undoped QDs. It is interesting noted that, with an increase in B2H6 pulse injection from 1 to 4 times, the integrated PL intensity was enhanced by a factor of 1.4 to 2.4 compared to that of the undoped QDs while no significant change in spectral shape was observable. This can be interpreted in terms of an increase in the number of holes with B-doping to the Ge core since the carrier recombination rates is proportional to the product of the number of electrons and holes confined in QD under weak photoexcitation.

Grain boundary structural transformation induced by co-segregation of aliovalent dopants

Impurity doping is a conventional but one of the most effective ways to control the functional properties of materials. In insulating materials, the dopant solubility limit is considerably low in general, and the dopants often segregate to grain boundaries (GBs) in polycrystals, which significantly alter their entire properties. However, detailed mechanisms on how dopant atoms form structures at GBs and change their properties remain a matter of conjecture. Here, we show GB structural transformation in α-Al2O3 induced by co-segregation of Ca and Si aliovalent dopants using atomic-resolution scanning transmission electron microscopy combined with density functional theory calculations. To accommodate large-sized Ca ions at the GB core, the pristine GB atomic structure is transformed into a new GB structure with larger free volumes. Moreover, the Si and Ca dopants form a chemically ordered structure, and the charge compensation is achieved within the narrow GB core region rather than forming broader space charge layers. Our findings give an insight into GB engineering by utilizing aliovalent co-segregation.

Effect of Impurity Scattering on Percolation of Bosonic Islands and Superconductivity in Fe Implanted NbN Thin Films.

A reentrant temperature dependence of the thermoresistivity between an onset local superconducting ordering temperature and a global superconducting transition at has been reported in disordered conventional 3-dimensional (3D) superconductors. The disorder of these superconductors is a result of either an extrinsic granularity due to grain boundaries, or of an intrinsic granularity ascribable to the electronic disorder originating from impurity dopants. Here, the effects of Fe doping on the electronic properties of sputtered NbN layers with a nominal thickness of 100 nm are studied by means of low-T/high- magnetotransport measurements. The doping of NbN is achieved via implantation of 35 keV Fe ions. In the as-grown NbN films, a local onset of superconductivity at is found, while the global superconducting ordering is achieved at , with a normal state resistivity . Moreover, upon Fe doping of NbN, is estimated, while and are measured to be 15.1 K and 13.5 K, respectively. In Fe:NbN, the intrinsic granularity leads to the emergence of a bosonic insulator state and the normal-metal-to-superconductor transition is accompanied by six different electronic phases characterized by a N-shaped T dependence of . The bosonic insulator state in a s-wave conventional superconductor doped with dilute magnetic impurities is predicted to represent a workbench for emergent phenomena, such as gapless superconductivity, triplet Cooper pairings and topological odd frequency superconductivity.

Hole-Doping to a Cu(I)-Based Semiconductor with an Isovalent Cation: Utilizing a Complex Defect as a Shallow Acceptor.

p-Type doping in Cu(I)-based semiconductors is pivotal for solar cell photoabsorbers and hole transport materials to improve the device performance. Impurity doping is a fundamental technology to overcome the intrinsic limits of hole concentration controlled by native defects. Here, we report that alkali metal impurities are prominent p-type dopants for the Cu(I)-based cation-deficient hole conductors. When the size mismatch with Cu+ in the host lattice is increased, these isovalent impurities are preferentially located at interstitial positions to interact with the constituent Cu cations, forming stable impurity-defect complexes. We demonstrate that the Cs impurity in γ-CuI semiconductors enhances hole concentration controllability for single crystals and thin films in the range of 1013-1019 cm-3. First-principles calculations indicate that the Cs impurity forms impurity-defect complexes that act as shallow acceptors leading to the increased p-type conductivity. This isovalent doping provides an approach for controlled doping into cation-deficient semiconductors through an interaction of impurities with native defects.

Microscopic probing of the doping effects of In ions in Fe3O4

Minute examination of local lattice structures in matter affected by impurity doping is of special importance for the development of functional materials. In order to obtain microscopic information on spinel ferrites, in the present work, we introduced nonmagnetic In3+ ions in Fe3O4 and probed their site selectivity and the doping effect on the local lattice structures and bulk magnetism by means of 57Fe Mössbauer spectroscopy and positron annihilation spectroscopies. The Mössbauer parameters of the area intensity and isomer shift (IS) show that In3+ ions predominantly reside in the tetrahedral A site, especially at low doping level. With increasing concentration of In ions, however, they gradually occupy the octahedral B site replacing Fe3+ ions. Along with the site information, the IS values confirmed that the introduced In ions squeeze the B-site Fe ions at their nearest neighbors. Supporting results were obtained from positron annihilation lifetime spectroscopy; positron lifetimes become shorter with increasing In concentration, signifying that the oxygen ions are pressed by the introduced In ions resulting in lowering the volume of the adjacent lattice vacancies. The results of Doppler broadening spectroscopy also support the squeezing effect; the positrons in the vacancies adjacent to In ions are more likely to annihilate with the inner shell electrons of the surrounding oxygen ions as a result of a reduction in the vacancy volume.

Co-doping of Phosphorous-Boron in-situ grown c-Si quantum dots/a-SiOx:H thin films on PET

Phosphorus-boron co-doped amorphous hydrogenated silicon oxide (a-SiOx:H) thin films containing crystalline silicon quantum dot (c-Si QD) were prepared via plasma enhanced chemical vapor deposition (PECVD) route on polyethylene terephthalate (PET) substrate. With the aim of optimizing the doping efficiency and effectively controlling the properties of the flexible thin films, the helium (He) and argon (Ar) mixed dilution was applied in the in-situ deposition process. A series of characterizations have been carried out for studying the variation in the properties of flexible P-B co-doped Si QD/a-SiOx:H thin films when tuning the He/Ar flow ratio. Successful P-B co-doping has been proved according to the redshift of photoluminescence (PL) peak. Experimental results also show that the QD size and the doping concentration are controllable when varying the He/Ar flow ratio, while the PL peaks are tunable with QD size. Furthermore, the mechanisms on film growth and impurity doping were illustrated in detail.

Doping-Mediated Size and Structure Tailoring of CaS Nanocrystals

Lanthanide-doped CaS crystals are extensively studied as light-emitting materials, but the investigation of CaS nanocrystals is largely lagging. This work reports a doping-induced modification of surface charge density and the local coordination environment in CaS:Ce nanocrystals, which offers precise control over the nanocrystal size (39–14 nm) and emission color (green to yellow). Our mechanistic investigations, corroborated by theoretical calculations, reveal a S-enriched nanocrystal surface that accommodates excess charges of the aliovalent lanthanide dopants. Owing to the altered surface structure, we identify a dark Ce state that selectively extinguishes light emission of the CaS:Ce nanocrystals. We demonstrate that the doping-induced surface effects can be offset by coating the CaS:Ce nanocrystals with an undoped CaS shell, giving rise to a shell-thickness-dependent emission. Our findings suggest that nanocrystals may respond differently to impurity dopants than the corresponding bulk crystals, thereby giving rise to extended control over materials properties for applications such as optical encryption.