Dopant Migration Research Articles

Metal-Oxide Nanoparticles with a Dopant-Segregation-Induced Core–Shell Structure: Gas Sensing Properties

Gas sensors made from high-surface area metal-oxide nanoparticles are susceptible to thermal degradation caused by grain growth and coalescence during service at high working temperature. Therefore, it is crucial to develop an effective strategy to control the rate of grain growth of these nanomaterials without compromising the sensitivity. This paper reports synthesis and characteristics of Al-doped SnO2 nanomaterials with a dopant-segregation-induced core (dopant-free)/shell (dopant-rich) structure to improve the material stability and optimize the gas sensing sensitivity. High-temperature (1100 °C) annealing of a polymer precursor derived Al-doped SnO2 results in the formation of an Al-rich shell because of dopant migration driven by strain and electrostatic interaction energy. Attributed to the solute drag effect, the SnO2 samples with an Al-rich shell show smaller crystallite size and a lower level of coalescence than the pure SnO2, leading to notable enhancement in H2 sensitivity (10×) for the SnO2 ...

Role of Transition Metal Identity in Suppression of Interlayer Migration in Layered Cathode Materials

As Li-ion batteries continue to dominate in the portable electronics and electric car industries we look to increase capacity by cycling the batteries at higher voltages. Alloying transition metals and dopants to cathode materials can aid in pushing the voltage limit beyond current capabilities. However, at higher voltages, problems like interlayer dopant migration become a serious detriment that result in phase transformations and material break down. Using first-principles methods, we have unraveled the electronic structure that depresses interlayer atom migration. Surprisingly, this electronic structure is largely a contribution of the local environment around the dopant and not simply a function of the dopant’s identity. Using this understanding, we propose design principles for managing interlayer atom migration in cathode materials, with specific implications for NCA, NMC, and Li-excess cathodes. Figure 1

Damage recovery and dopant migration of Eu+ ion implanted KTiOAsO4 crystals

Damage formation and annealing of Eu implanted KTiOAsO4 crystals is investigated. Ion implantation was done with 400 keV Eu2+ ions and an ion fluence of 5 × 1015 ions/cm2 at room temperature. The damage of as-implanted and annealed samples was analysed by Rutherford backscattering spectrometry in channelling configuration using 2.1 MeV He+ ions. After implantation, a thick RBS-amorphous layer is obtained. The damage in the Eu2+ implanted KTiOAsO4 layer decreases significantly after annealing at 700° C for 30 min in Ar atmosphere. Interestingly, the Eu peak firstly migrated to the surface after annealing at 600° C for 30 min, and then appeared narrower after annealing at 700° C for 30 min. Furthermore, arsenic loss by evaporation and titanium enrichment at the surface are observed during the annealing process.

Crystal plane dependent dopant migration that boosts catalytic oxidation

CeO2 rods with {110} facets and cubes with {100} facets were utilized as catalyst supports to probe the effect of crystallographic facets on the iron species and the structure-dependent catalytic performance.

Interface Engineering of Mn-Doped ZnSe-Based Core/Shell Nanowires for Tunable Host-Dopant Coupling.

Transition metal ion doped one-dimensional (1-D) nanocrystals (NCs) have advantages of larger absorption cross sections and polarized absorption and emissions in comparison to 0-D NCs. However, direct synthesis of doped 1-D nanorods (NRs) or nanowires (NWs) has proven challenging. In this study, we report the synthesis of 1-D Mn-doped ZnSe NWs using a colloidal hot-injection method and shell passivation for core/shell NWs with tunable optical properties. Experimental results show optical properties of the NWs are controlled by the composition and thickness of the shell lattice. It was found that both the host-Mn energy transfer and Mn-Mn coupling are strongly dependent on the type of alloy at the interface of doped core/shell NWs. For Mn-doped type I ZnSe/ZnS core/shell NWs, the ZnS shell passivation can enhance florescence quantum yield with little effect on the location of the incorporated Mn dopant due to the identical cationic Zn2+ site available for Mn dopants throughout the core/shell NWs. However, for Mn-doped quasi type II ZnSe/CdS NWs and ZnSe/CdS/ZnS core/shell NWs, the cation alloying (Zn1-xCdxS(e)) can lead to metal dopant migration from the core to the alloyed interface and tunable host-dopant energy transfer efficiencies and Mn-Mn coupling. As a result, a tunable dual-band emission can be achieved for the doped NWs with the cation-alloyed interface. The interfacial alloying mediated energy transfer and Mn-Mn coupling provides a method to control the optical properties of the doped 1-D core/shell NWs.

Effect of Cooling Rate on Dopant Spatial Localization and Phase Transformation in Cu‐Doped Y‐Stabilized ZrO2 Nanopowders

The effect of calcination temperature (TC = 500–1000 °C) and cooling rate on the dopant distribution in Cu‐doped Y‐stabilized ZrO2 nanopowders is studied. The powders are produced by co‐precipitation technique and investigated by attenuated total reflection, UV‐vis diffuse reflectance, electron paramagnetic resonance, and transmission electron microscopy methods. The cooling rate is found to affect the amount of Cu substances on grain surface, the powders subjected to fast cooling (quenching) showed higher amount of Cu‐related complexes on the grains’ surface than their counterparts cooled with furnace after calcination. It is observed that Cu impurities diffuse inside ZrO2 grains from Cu‐related surface substances when TC < 800 °C. This process is accompanied by the enhancement of 275‐nm absorption band caused by oxygen vacancies formation. For TC > 800 °C, outward migration of Cu dopants takes place. Simultaneously, the intensity of 275‐nm absorption band decreases, the monoclinic ZrO2 phase forms and its contribution rises with TC. It is proposed that monoclinic phase formation is caused by the replacement of Cu atoms from lattice sites to interstitials leading to an appearance of the channels for Y out‐diffusion via cation vacancies and destabilization of ZrO2 tetragonal phase.

Nanoscale insights into doping behavior, particle size and surface effects in trivalent metal doped SnO2

Despite considerable research, the location of an aliovalent dopant into SnO2 nanoparticles is far to be clarified. The aim of the present study on trivalent lanthanide doped SnO2 is to differentiate between substitutional versus interstitial and surface versus bulk doping, delineate the bulk and surface defects induced by doping and establish an intrinsic dopant distribution. We evidence for the first time a complex distribution of intrinsic nature composed of substitutional isolated, substitutional associates with defects as well as surface centers. Such multi-modal distribution is revealed for Eu and Sm, while Pr, Tb and Dy appear to be distributed mostly on the SnO2 surface. Like the previously reported case of Eu, Sm displays a long-lived luminescence decaying in the hundreds of ms scale which is likely related to a selective interaction between the traps and the substitutional isolated center. Analyzing the time-gated luminescence, we conclude that the local lattice environment of the lattice Sn is not affected by the particle size, being remarkably similar in the ~2 and 20 nm particles. The photocatalytic measurements employed as a probe tool confirm the conclusions from the luminescence measurements concerning the nature of defects and the temperature induced migration of lanthanide dopants.

Migration of Mg and other interstitial metal dopants in GaN

The minimum energy paths for the migration of interstitial Mg in wurtzite GaN are studied through density functional calculations. The study also comprises Li, Na, and Be dopants to examine the dependence on size and charge of the dopant species. In all cases considered, the impurities diffuse like ions without any tendency of localizing charge. Li, Mg, and to some extent Na, diffuse almost isotropically in GaN, with average diffusion barriers of 1.1, 2.1, and 2.5 eV, respectively. Instead Be shows a marked anisotropy with energy barriers of 0.76 and 1.88 eV for diffusion paths perpendicular and parallel to the c‐axis. The diffusion barrier generally increases with ionic charge and ionic radius, but their interplay is not trivial. The calculated migration barrier for Mg is consistent with the values estimated in a recent emission channeling experiment.

Controlled Dopant Migration in CdS/ZnS Core/Shell Quantum Dots.

The physical properties of a doped quantum dot (QD) are strongly influenced by the dopant site inside the host lattice, which determines the host-dopant coupling from the overlap between the dopant and exciton wave functions of the host lattice. Although several synthetic methodologies have been developed for introducing dopants inside the size-confined semiconductor nanocrystals, the controlled dopant-host lattice coupling by dopant migration is still unexplored. In this work, the effect of lattice mismatch of CdS/ZnS core/shell QDs on Mn(II) dopant behavior was studied. It was found that the dopant migration toward the alloyed interface of core/shell QDs is a thermodynamically driven process to minimize the lattice strain within the nanocrystals. The dopant migration rate could be represented by the Arrhenius equation and therefore can be controlled by the temperature and lattice mismatch. Furthermore, the energy transfer between host CdS QDs and dopants can be finely turned in a wide range by dopant migration toward the alloyed interface during ZnS shell passivation, which provides an efficient method to control both the number of the emission band and the ratio of the emission from the host lattice and dopant ions.

High-Power Growth-Robust InGaAs/InAlAs Terahertz Quantum Cascade Lasers.

We report on high-power terahertz quantum cascade lasers based on low effective electron mass InGaAs/InAlAs semiconductor heterostructures with excellent reproducibility. Growth-related asymmetries in the form of interface roughness and dopant migration play a crucial role in this material system. These bias polarity dependent phenomena are studied using a nominally symmetric active region resulting in a preferential electron transport in the growth direction. A structure based on a three-well optical phonon depletion scheme was optimized for this bias direction. Depending on the sheet doping density, the performance of this structure shows a trade-off between high maximum operating temperature and high output power. While the highest operating temperature of 155 K is observed for a moderate sheet doping density of 2 × 1010 cm–2, the highest peak output power of 151 mW is found for 7.3 × 1010 cm–2. Furthermore, by abutting a hyperhemispherical GaAs lens to a device with the highest doping level a record output power of 587 mW is achieved for double-metal waveguide structures.

Materials science: Polymers make charge flow easy.

Organic semiconductor devices require good electrical contacts with conducting materials, but such contacts are often inefficient. An approach that tackles this problem will enable a wide range of applications. See Letter p.536 Electronic and optoelectronic devices made from organic polymers can be produced via solution processing, which is inexpensive, offers a high degree of design control and is especially suitable for producing flexible materials. An outstanding challenge is to make good 'ohmic' electrical contact with metal connections. This requires polymers with high charge doping content, but it is difficult to obtain highly doped polymer films from solution that retain their doping charges and are sufficiently stable. Peter Ho and colleagues present a general approach to produce polymers with stable, high doping content, by including a self-compensation mechanism that involves covalently bonded counter ions to block the migration of dopants. They apply this to a range of polymers and demonstrate devices including high-performance light-emitting diodes and ambipolar field-effect transistors.

Doped polymer semiconductors with ultrahigh and ultralow work functions for ohmic contacts.

To make high-performance semiconductor devices, a good ohmic contact between the electrode and the semiconductor layer is required to inject the maximum current density across the contact. Achieving ohmic contacts requires electrodes with high and low work functions to inject holes and electrons respectively, where the work function is the minimum energy required to remove an electron from the Fermi level of the electrode to the vacuum level. However, it is challenging to produce electrically conducting films with sufficiently high or low work functions, especially for solution-processed semiconductor devices. Hole-doped polymer organic semiconductors are available in a limited work-function range, but hole-doped materials with ultrahigh work functions and, especially, electron-doped materials with low to ultralow work functions are not yet available. The key challenges are stabilizing the thin films against de-doping and suppressing dopant migration. Here we report a general strategy to overcome these limitations and achieve solution-processed doped films over a wide range of work functions (3.0-5.8 electronvolts), by charge-doping of conjugated polyelectrolytes and then internal ion-exchange to give self-compensated heavily doped polymers. Mobile carriers on the polymer backbone in these materials are compensated by covalently bonded counter-ions. Although our self-compensated doped polymers superficially resemble self-doped polymers, they are generated by separate charge-carrier doping and compensation steps, which enables the use of strong dopants to access extreme work functions. We demonstrate solution-processed ohmic contacts for high-performance organic light-emitting diodes, solar cells, photodiodes and transistors, including ohmic injection of both carrier types into polyfluorene-the benchmark wide-bandgap blue-light-emitting polymer organic semiconductor. We also show that metal electrodes can be transformed into highly efficient hole- and electron-injection contacts via the self-assembly of these doped polyelectrolytes. This consequently allows ambipolar field-effect transistors to be transformed into high-performance p- and n-channel transistors. Our strategy provides a method for producing ohmic contacts not only for organic semiconductors, but potentially for other advanced semiconductors as well, including perovskites, quantum dots, nanotubes and two-dimensional materials.

Defect-Rich Dopant-Free ZrO2 Nanostructures with Superior Dilute Ferromagnetic Semiconductor Properties.

Control of the spin degree of freedom of an electron has brought about a new era in spin-based applications, particularly spin-based electronics, with the potential to outperform the traditional charge-based semiconductor technology for data storage and information processing. However, the realization of functional spin-based devices for information processing remains elusive due to several fundamental challenges such as the low Curie temperature of group III-V and II-VI semiconductors (<200 K), and the low spin-injection efficiencies of existing III-V, II-VI, and transparent conductive oxide semiconductors in a multilayer device structure, which are caused by precipitation and migration of dopants from the host layer to the adjacent layers. Here, we use catalyst-assisted pulsed laser deposition to grow, for the first time, oxygen vacancy defect-rich, dopant-free ZrO2 nanostructures with high TC (700 K) and high magnetization (5.9 emu/g). The observed magnetization is significantly greater than both doped and defect-rich transparent conductive oxide nanomaterials reported to date. We also provide the first experimental evidence that it is the amounts and types of oxygen vacancy defects in, and not the phase of ZrO2 that control the ferromagnetic order in undoped ZrO2 nanostructures. To explain the origin of ferromagnetism in these ZrO2 nanostructures, we hypothesize a new defect-induced bound polaron model, which is generally applicable to other defect-rich, dopant-free transparent conductive oxide nanostructures. These results provide new insights into magnetic ordering in undoped dilute ferromagnetic semiconductor oxides and contribute to the design of exotic magnetic and novel multifunctional materials.

Effects of Electrostatically Doped Source/Drain and Ferroelectric Gate Oxide on Subthreshold Swing and Impact Ionization Rate of Strained-Si-on-Insulator Tunnel Field-Effect Transistors

This letter reports an electrostatically doped source/drain (EDSD) ferroelectric strained-Si-on-insulator (Fe-SSOI) tunnel field-effect transistor (TFET) with the subthreshold swing (SS) as low as 10 mV/dec for sub-30 nm applications. The proposed device, named as EDSD Fe-SSOI TFET in this report, uses two metal electrodes of Pt and Hf placed at the two sides of the gate on a thin intrinsic (dopingless) strained-Si layer to convert the undoped strained-Si regions below the Pt and Hf electrodes into the respective p+ source and n + drain regions (by plasma charge phenomenon) of the TFET using the ferroelectric stacked gate oxides. While the plasma charge concept provides the abrupt source (drain)–channel junction by avoiding dopant migration from the source (drain) to channel, the intrinsic channel makes the proposed TFET device free from the random dopant fluctuation-related problems. The combined effects of the ferroelectric stacked gate oxides and the strain in intrinsic-Si channel of the proposed TFET structure are observed to result in a significant improvement in the SS as well as the drain current of the proposed EDSD Fe-SSOI TFET under consideration.

Theoretical Study of the Stoichiometric and Reduced Ce-Doped TiO2 Anatase (001) Surfaces

The effects of Ce doping (2.6%) on the oxygen vacancy (VO) formation energy (Ef) and the electronic structure of the anatase TiO2(001) surface were studied by means of periodic density functional calculations within the PBE and PBE + U approaches. Several situations were considered for VO formation, differing in terms of the position in relation to the dopant site (at the surface and subsurface atomic layers). The vacancy energy of formation is almost always lower for the surface than for the bulk, but the difference is still larger with Ce dopant in the subsurface layers. Nevertheless, the Ce-for-Ti substitution is more stable at the outermost layers, indicating thermodynamically favorable dopant migration toward the oxide surface. The PBE + U approach provides a physically meaningful description of localized d and f electrons in Ti3+ and Ce3+ species, respectively. Moreover, fully localized spin (simple and split) or partially localized spin solutions are found within a ∼0.5 eV range. Not unexpectedly, ...

Importance of growth direction in mid-infrared quantum cascade lasers

We report on the effect of growth direction on the performance of mid-infrared Quantum Cascade lasers. The design used has a symmetric active core, capable of operating under both negative and positive polarities, which allows to test for residual growth asymmetries such as interface roughness and dopant migration. Calculations of scattering lifetimes from interface roughness and ionized impurities suggest a dominant contribution from the former, with devices biased positively averaging ∼15% larger broadening and ∼50% shorter upper state lifetime than negatively biased devices. Experimental results for positively biased devices show at least 30% larger broadening and 35% lower electroluminescence peak intensity than those biased negatively, in good agreement with the modeling results.

High Voltage Pyrophosphate Cathodes for Li- and Na-Ion Batteries: Atomic-Scale Insights into Defects and Diffusion

Major advances in rechargeable batteries for electronics, electric vehicles and grid storage requires the development of new materials. Recently, it has been proposed that the pyrophosphate framework may provide a new platform for important electrode research. Li2FeP2O7 displays reversible reaction at 3.5 V vs Li/Li+ without nanosizing or carbon coating, which is the highest voltage for any Fe-based phosphate cathode [1]. There is growing interest in sodium-ion batteries largely due to the natural abundance of sodium and cost issues for grid-storage. The sodium pyrophosphate, Na2FeP2O7, has been found to be electrochemically active (3V vs Na/Na+) [2], which is the first pyrophosphate-type of cathode for Na-ion batteries.This presentation highlights recent studies of both Li2FeP2O7 and Na2FeP2O7 using atomistic simulation techniques [3] to provide detailed insights into defect, dopant and ion migration properties relevant to their electrochemical behaviour [4]. Li/Fe and Na/Fe antisites were found to be the most favourable intrinsic defects suggesting significant disorder, which would be sensitive to synthesis conditions. In contrast to 1D diffusion in LiFePO4, fast Li+ transport in Li2FeP2O7 is predicted to be through a 2D network. Interestingly, the Na2FeP2O7 structure appears to support quasi-3D Na+ diffusion. Hence, a significant result is that alkali-ion diffusion within this framework is found to be either 2D or 3D with low energy barriers, which are important for good rates of charge/discharge.

Dopant migration effects in terahertz quantum cascade lasers

We demonstrate that dopant migration and the resulting impurity scattering can strongly influence the performance of GaAs/Al0.15Ga0.85As terahertz quantum cascade lasers. A nominally symmetric structure allows us to compare the negative and positive bias behavior of the very same device. Dopants, migrated towards the upper laser level during sample growth, degrade the lasing performance due to enhanced impurity scattering rates. The consequences are a higher threshold current (+30%) and reduced optical output power (–29%) in the affected operating direction. This polarity dependent performance is reversed in an asymmetrically doped sample, which imitates the migration of dopants against the growth direction.

Stabilizing nanocrystalline grains in ceramic-oxides

Nanocrystalline ceramic-oxides are prone to grain growth rendering their highly attractive properties practically unusable. Using atomistic simulations of ceria as a model material system, we elucidate a framework to design dopant-pinned grain boundaries that prevent this grain growth. While in metallic systems it has been shown that a large mismatch between host and dopant atomic sizes prevents grain growth, in ceramic-oxides we find that this concept is not applicable. Instead, we find that dopant-oxygen vacancy interaction, i.e., dopant migration energy in the presence of an oxygen vacancy, and dopant-oxygen vacancy binding energy are the controlling factors in grain growth. Our prediction agrees with and explains previous experimental observations.

Configurations, electronic properties, and diffusion of carbon and nitrogen dopants in rutile TiO2: A density functional theory study

Doping of TiO${}_{2}$ with nonmetal atoms is known to improve the photoconversion efficiency of the material. Here, we use first-principles calculations to describe the atomic-scale details of migration and configurational changes of typical dopants and oxygen-related native defects in rutile TiO${}_{2}$. The complex pathways for transformations of carbon and nitrogen dopants include structures, which, though very close in energy, have different effects on the electronic properties of the host system in terms of impurity-related gap states. We also find that, because of relatively low diffusion barriers, moderate annealing can activate the migration of impurities and native defects and lead to defect-induced transformations of dopants. Overall, the results are relevant to the dynamics of C and N dopants in rutile TiO${}_{2}$ and to the performance of the material in photocatalytic and photovoltaic applications.