Native Point Defects Research Articles

Novel Strategy for the Synthesis of Ultra-Stable Single-Site Mo-ZSM-5 Zeolite Nanocrystals.

The current energy transition presents many technological challenges, such as the development of highly stable catalysts. Herein, we report a novel "top-down" synthesis approach for preparation of a single-site Mo-containing nanosized ZSM-5 zeolite which has atomically dispersed framework-molybdenum homogenously distributed through the zeolite crystals. The introduction of Mo heals most of the native point defects in the zeolite structure resulting in an extremely stable material. The important features of this single-site Mo-containing ZSM-5 zeolite are provided by an in-depth spectroscopic and microscopic analysis. The material demonstrates superior thermal (up to 1000 °C), hydrothermal (steaming), and catalytic (converting methane to hydrogen and higher hydrocarbons) stability, maintaining the atomically disperse Mo, structural integrity of the zeolite, and preventing the formation of silanols.

Correlation of native point defects and photocatalytic activity of annealed ZnO nanoparticle studied by electron spin resonance and photoluminescence emission

In this paper, we investigate the origin of point defects revealed by electron spin resonance (ESR) and photoluminescence (PL) emission in correlation with the photocatalytic activity of ZnO nanocrystals subjected to thermal annealing at various temperatures. Two ESR signals at g ∼ 1.96 and ∼2.003 were consistently observed in all annealed ZnO samples. However, their relative intensities have changed, indicating that the point defect densities were influenced by the annealing temperature. Interestingly, when doping nanoZnO with Cr3+, the Q-band ESR measurements at T = 100 K did show that the g ∼ 1.96 signal was completely passivated, suggesting that the origin of the signal lies in the electrons located near the conduction band, i.e. at a shallow-donor level. The intensity of the g ∼ 2.003 signal decreased by rising the annealing temperature, and this is attributed to the depopulation of zinc interstitials through the thermally induced migration process and/or recombination with the zinc vacancies. The green PL emission line at ∼520 nm, which is dominant in the 700 °C annealed ZnO sample, shows a correlation with the ESR signal at g ∼ 1.96, whose origin is attributed to the radiative transition of the electron from the shallow donor level to the singly ionized zinc vacancy. Furthermore, the high density of the shallow donor electron states was found to be primarily responsible for the high photocatalytic activity in the degradation of methylene blue.

Native Point Defects in Monolayer Hexagonal Boron Phosphide from First Principles

In this paper, we have investigated the electronic and magnetic properties of four types of native defects under neutral and charged states in a hexagonal boron phosphide (h-BP) monolayer, including boron vacancy ( $$ V_{\rm{B}} ) $$ , phosphorus vacancy ( $$ V_{\rm{P}} ) $$ , boron on the phosphorus site ( $$ B_{\rm{P}} ) $$ and phosphorus on the boron site ( $$ P_{\rm{B}} ) $$ within the framework of the density functional theory. For the four types of defects, various charge states were investigated, and only 0 and 1 + charge states for all defects are stable within the electronic chemical potential range (i.e. Fermi level range). It is found that $$ B_{\rm{P}} $$ with the smallest defect formation energy is the most stable defect under both phosphorus-rich and -poor conditions in the whole range of electronic chemical potential. $$ V_{\rm{P}} $$ and $$ P_{\rm{B}} $$ are found to be shallow donors (i.e. 1 +/0) but could not be effectively introduced into the h-BP monolayer due to a rather high formation energy, while $$ V_{\rm{B}} $$ and $$ B_{\rm{P}} $$ are found to be holes trap centers. Especially, $$ B_{\rm{P}} $$ with a low defect formation energy, will be produced easily and seriously affect the p-type doping efficiency and conductivity of h-BP. Additionally, $$ V_{\rm{B}} $$ and $$ V_{\rm{P}} $$ induce a nonzero magnetic moment while $$ P_{\rm{B}} $$ and $$ B_{\rm{P}} $$ show non-magnetic nature in the h-BP monolayer.

Wurtzite quantum well structures under high pressure

Quantum well systems based on semiconductors with the wurtzite crystalline structure have found widespread applications in photonics and optoelectronic devices, such as light-emitting diodes, laser diodes, or single-photon emitters. In these structures, the radiative recombination processes can be affected by (i) the presence of strain and polarization-induced electric fields, (ii) quantum well thickness fluctuations and blurring of a well–barrier interface, and (iii) the presence of dislocations and native point defects (intentional and unintentional impurities). A separate investigation of these phenomena is not straightforward since they give rise to similar effects, such as a decrease of luminescence efficiency and decay rate, enhancement of the Stokes shift, and strong blueshift of the emission with increasing pump intensity. In this Perspective article, we review the usefulness of measurements of the quantum well luminescence as a function of the hydrostatic pressure for both scientific research and the development of light-emitting technologies. The results presented here show that high-pressure investigations combined with ab initio calculations can identify the nature of optical transitions and the main physical factors affecting the radiative efficiency in quantum well systems. Finally, we will discuss an outlook to the further possibilities to gain new knowledge about the nature of recombination processes in quantum wells using high-pressure spectroscopy.

Effect of carbon-doping on structural and dielectric properties of zinc oxide

In this paper, Carbon-doped Zinc Oxide (C-ZnO) samples were prepared using the solid-state reaction method. The influence of carbon-doping on the structural and dielectric properties of ZnO samples was studied. The shift in the highest peak position (101) in XRD patterns of carbon-doped samples was observed. The Raman peak at 581[Formula: see text]cm[Formula: see text] in undoped ZnO was shifted and broadened in carbon-doped ZnO samples. The ZnO samples doped with carbon show higher values of dielectric constant ([Formula: see text] at 1[Formula: see text]kHz) compared to pure ZnO([Formula: see text] at 1[Formula: see text]kHz) which was due to increase in native point defects in the samples. The ac conductivity ([Formula: see text]) value of the carbon-doped sample was enhanced by 103 times for ((ZnO)[Formula: see text] [Formula: see text]) sample.

Ferromagnetic behavior of native point defects and vacancy-clusters in ZnO studied by first principle calculation

The origin of room temperature ferromagnetism in undoped ZnO is still a question of debate. Experimental and theoretical findings are inconclusive as to the predominant contributor for the magnetic behavior of undoped ZnO. First principle calculation pseudopotential method was used to systematically determine the relaxed atomic geometry, the formation energies and the magnetic properties of the native point defects (vacancies, interstitials and antisites), and vacancy clusters (VZnVO, VZn − 2VO and 2VZn − VO) in ZnO. The results show that ZnO cells consisting of the VZn and the Oi have non-zero magnetic moments, energetically favoring ferromagnetic states and close-to-room-temperature Curie temperatures (294 K). VZn and Oi are also characterized by their low formation energies, in particular in the case of n-type (i.e. Fermi level close to the conduction band minimum) and O-rich conditions. The energy differences between the ferromagnetic state and anti-ferromagnetic state for VZn and Oi are larger than kT at room temperature but still relatively small (∼34 meV). Although VZn and Oi would contribute for the room temperature ferromagnetism, the ferromagnetism states would not be robustly stable for thermal excitation to the anti-ferromagnetic states.

Native point defects in mono and bilayer phosphorene

We study the stability and electronic properties of intrinsic point defects, vacancy and self-interstitial, in mono- and bi-layer phosphorene. We calculate the formation energies, quasiparticle defect states and charge transition levels (CTLs) of these defects using \textit{ab initio} density functional theory (DFT) and GW approximation to the electron self-energy. Using the DFT + GW two paths formalism for studying interstitial in monolayer phosphorene, we show that with the inclusion of electrostatic corrections CTLs can be calculated reliably. Our calculations show that all the native point defects have low formation energies 0.9-1.6 eV in neutral state. Furthermore, we find that vacancy in phosphorene behaves as an acceptor-like defect which can explain the p-type conductivity in phosphorene. On the other hand, interstitial can show both acceptor- and donor-like behaviour.

Intrinsic Point Defects and Dopants Ce3+ in SrLiAl3N4: Thermodynamic and Spectral Properties from First Principles

Lanthanide-doped SrLiAl3N4 are among the most promising phosphor materials for solid-state lighting due to their superior luminescence properties. Native point defects occur naturally during their ...

Photoluminescence of ZnO Nanowires: A Review.

One-dimensional ZnO nanostructures (nanowires/nanorods) are attractive materials for applications such as gas sensors, biosensors, solar cells, and photocatalysts. This is due to the relatively easy production process of these kinds of nanostructures with excellent charge carrier transport properties and high crystalline quality. In this work, we review the photoluminescence (PL) properties of single and collective ZnO nanowires and nanorods. As different growth techniques were obtained for the presented samples, a brief review of two popular growth methods, vapor-liquid-solid (VLS) and hydrothermal, is shown. Then, a discussion of the emission process and characteristics of the near-band edge excitonic emission (NBE) and deep-level emission (DLE) bands is presented. Their respective contribution to the total emission of the nanostructure is discussed using the spatial information distribution obtained by scanning transmission electron microscopy−cathodoluminescence (STEM-CL) measurements. Also, the influence of surface effects on the photoluminescence of ZnO nanowires, as well as the temperature dependence, is briefly discussed for both ultraviolet and visible emissions. Finally, we present a discussion of the size reduction effects of the two main photoluminescent bands of ZnO. For a wide emission (near ultra-violet and visible), which has sometimes been attributed to different origins, we present a summary of the different native point defects or trap centers in ZnO as a cause for the different deep-level emission bands.

Atomic-scale defects in the two-dimensional ferromagnet CrI3 from first principles

The family of atomically thin magnets holds great promise for a number of prospective applications in magneto-optoelectronics, with CrI3 arguably being its most prototypical member. However, the formation of defects in this system remains unexplored to date. Here, we investigate native point defects in monolayer CrI3 by means of first-principles calculations. We consider a large set of intrinsic impurities and address the atomic structure, thermodynamic stability, diffusion and aggregation tendencies as well as local magnetic moments. Under thermodynamic equilibrium, the most stable defects are found to be either Cr or I atomic vacancies along with their complexes, depending on the chemical potential conditions. These defects are predicted to be quite mobile at room and growth temperatures, and to exhibit a strong tendency to agglomerate. In addition, our calculations indicate that the deviation from the nominal stoichiometry largely impacts the magnetic moments, and the defect-induced lattice distortions can drive local ferromagnetic-to-antiferromagnetic phase transitions. Overall, this work portrays a comprehensive picture of intrinsic point defects in monolayer CrI3 from a theoretical perspective.

Deep level defects and cation sublattice disorder in ZnGeN2

III-nitrides have revolutionized lighting technology and power electronics. Expanding the nitride semiconductor family to include heterovalent ternary nitrides opens up new and exciting opportunities for device design that may help overcome some of the limitations of the binary nitrides. However, the more complex cation sublattice also gives rise to new interactions with both native point defects and defect complexes that can introduce disorder on the cation sublattice. Here, depth-resolved cathodoluminescence spectroscopy and surface photovoltage spectroscopy measurements of defect energy levels in ZnGeN2 combined with transmission electron microscopy and x-ray diffraction reveal optical signatures of mid-gap states that can be associated with cation sublattice disorder. The energies of these characteristic optical signatures in ZnGeN2 thin films grown by metal–organic chemical vapor deposition are in good agreement with multiple, closely spaced band-like defect levels predicted by density functional theory. We correlated spatially resolved optical and atomic composition measurements using spatially resolved x-ray photoelectron spectroscopy with systematically varied growth conditions on the same ZnGeN2 films. The resultant elemental maps vs defect spectral energies and intensities suggest that cation antisite complexes (ZnGe–GeZn) form preferentially vs isolated native point defects and introduce a mid-gap band of defect levels that dominate electron–hole pair recombination. Complexing of ZnGe and GeZn antisites manifests as disorder in the cation sub-lattice and leads to the formation of wurtzitic ZnGeN2 as indicated by transmission electron microscopy diffraction patterns and x-ray diffraction reciprocal space maps. These findings emphasize the importance of growth and processing conditions to control cation place exchange.

Ferromagnetic anisotropy in scandium-doped AlN hierarchical nanostructures

We experimentally fabricated uniformly distributed single-, double- and six-sided Sc-doped AlN (AlN:Sc) nanodendrites by a plasma-assisted direct current arc discharge method. The crystal structure, composition, morphological and microstructures were characterized, and the physical mechanisms governing the formation of AlN:Sc nanodendrites were examined. It was found that the morphology and the crystal orientation of the AlN nanodendrites can be controlled by tuning the N2 pressure. The incorporation of Sc induces native point defects in AlN that contribute to the growth of hierarchical structures and affect the room-temperature photoluminescence. The AlN:Sc nanodendrites exhibit room-temperature anisotropic ferromagnetism, and this is strongly depend on the orientation of the nanodendrites. This work provides a facile strategy to fabricate complex hierarchical AlN:Sc nanostructures with manipulated magnetic properties which may find promising applications in spintronic nanodevices.

Extrapolated Defect Transition Level in Two-Dimensional Materials: The Case of Charged Native Point Defects in Monolayer Hexagonal Boron Nitride.

Defect formation energy as well as the charge transition level (CTL) plays a vital role in understanding the underlying mechanism of the effect of defects on material properties. However, the accurate calculation of charged defects, especially for two-dimensional materials, is still a challenging topic. In this paper, we proposed a simplified scheme to rescale the CTLs from the semilocal to the hybrid functional level, which is time-saving during the charged defect calculations. Based on this method, we systematically calculated the formation energy of four kinds of intrinsic point defects in two-dimensional hexagonal boron nitride (2D h-BN) by uniformly scaling the supercells by which we found a time-saving method to obtain the "special vacuum size" (Komsa, H.-P.; Berseneva, N.; Krasheninnikov, A. V.; Nieminen, R. M. Phys. Rev. X, 2014, 4, 031044). Native defects including nitrogen vacancy (VN), boron vacancy (VB), nitrogen atom anti-sited on boron position (NB), and boron atom anti-sited on nitrogen position (BN) were calculated. The reliability of our scheme was verified by taking VN as a probe to conduct the hybrid functional calculation, and the rescaled CTL is within the acceptable error range with the pure HSE results. Based on the results of CTLs, all the native point defects in the h-BN monolayer act as hole or electron trap centers under certain conditions and would suppress the p- or n-type electrical conduction of h-BN-based devices. Our rescale method is also suitable for other materials for defect charge transition level calculations.

Soft Lattice and Defect Covalency Rationalize Tolerance of β-CsPbI3 Perovskite Solar Cells to Native Defects.

Although all-inorganic metal halide perovskites (MHPs) have shown tremendous improvement, they are still inferior to the hybrid organic-inorganic MHPs in efficiency. Recently, a conceptually new β-CsPbI3 perovskite reached 18.4 % efficiency combined with good thermodynamic stability at ambient conditions. We use ab initio non-adiabatic molecular dynamics to show that native point defects in β-CsPbI3 are generally benign for nonradiative charge recombination, regardless of whether they introduce shallow or deep trap states. These results indicate that MHPs do not follow the simple models used to explain defect-mediated charge recombination in the conventional semiconductors. The strong tolerance is due to the softness of the perovskite lattice, which permits separation of electrons and holes upon defect formation, and only allows carriers to couple to the low-frequency vibrations. Both factors decrease notably the non-adiabatic coupling and slow down the dissipation of energy to heat.

Soft Lattice and Defect Covalency Rationalize Tolerance of β‐CsPbI3 Perovskite Solar Cells to Native Defects

AbstractAlthough all‐inorganic metal halide perovskites (MHPs) have shown tremendous improvement, they are still inferior to the hybrid organic–inorganic MHPs in efficiency. Recently, a conceptually new β‐CsPbI3 perovskite reached 18.4 % efficiency combined with good thermodynamic stability at ambient conditions. We use ab initio non‐adiabatic molecular dynamics to show that native point defects in β‐CsPbI3 are generally benign for nonradiative charge recombination, regardless of whether they introduce shallow or deep trap states. These results indicate that MHPs do not follow the simple models used to explain defect‐mediated charge recombination in the conventional semiconductors. The strong tolerance is due to the softness of the perovskite lattice, which permits separation of electrons and holes upon defect formation, and only allows carriers to couple to the low‐frequency vibrations. Both factors decrease notably the non‐adiabatic coupling and slow down the dissipation of energy to heat.

Influence of oxygen-rich and zinc-rich conditions on donor and acceptor states and conductivity mechanism of ZnO films grown by ALD—Experimental studies

Understanding the origin of the strong difference of electrical parameters between as grown and annealed undoped ZnO films prepared at a temperature range of 100–200 °C by thermal atomic layer deposition is essential for their future applications. In this paper, we show that the conductivity drop by up to 4 orders of magnitude as effect of post-growth annealing is accompanied by multiple simultaneous effects like a two orders of magnitude decrease of hydrogen impurity content, a strong width reduction of the luminescence peaks, and an increase of crystallite sizes influencing the carrier scattering. We show that a level of structural and optical improvement as well as the final electrical parameters of annealed films strongly depend on the previously employed growth temperature, which is related to transition from oxygen- to zinc-rich conditions influencing a type and concentration of native point defects. The growth temperature does not only influence the bandgap energy but also the binding energies of existing donors and the relative ratio between the number of donors and acceptors; hence, it determines the final electrical characteristics of the films. This means that electrical properties of undoped ZnO-atomic layer deposition films can be tuned by native defects engineering.

Low-frequency lattice phonons in halide perovskites explain high defect tolerance toward electron-hole recombination.

Low-cost solution-based synthesis of metal halide perovskites (MHPs) invariably introduces defects in the system, which could form Shockley-Read-Hall (SRH) electron-hole recombination centers detrimental to solar conversion efficiency. Here, we investigate the nonradiative recombination processes due to native point defects in methylammonium lead halide (MAPbI3) perovskites using ab initio nonadiabatic molecular dynamics within surface-hopping framework. Regardless of whether the defects introduce a shallow or deep band state, we find that charge recombination in MAPbI3 is not enhanced, contrary to predictions from SRH theory. We demonstrate that this strong tolerance against defects, and hence the breakdown of SRH, arises because the photogenerated carriers are only coupled with low-frequency phonons and electron and hole states overlap weakly. Both factors appreciably decrease the nonadiabatic coupling. We argue that the soft nature of the inorganic lattice with small bulk modulus is key for defect tolerance, and hence, the findings are general to other MHPs.

An Ab Initio Perspective on the Key Defects of CsCu5Se3, a Possible Material for Optoelectronic Applications

We report herein a study of the native point defects of a newly synthesized chalcogenide material, CsCu5Se3, using an ab initio approach (DFT + U, hybrid functional) to assess the electronic proper...

Critical effect of carbon vacancies on the reverse water gas shift reaction over vanadium carbide catalysts

Experimental and theoretical evidences show that carbon vacancies determine the catalytic behavior of vanadium carbides in the CO2 conversion to CO via the Reverse Water Gas Shift (RWGS) reaction. Two VCx samples, one mostly containing stoichiometric VC and the other being C deficient, mainly V8C7, were synthesized, characterized, and studied. The samples show different CO2 adsorption heats, which correlate with those calculated using Density Functional Theory (DFT) on suitable models. The sample containing more V8C7 shows a higher CO2 conversion and CO selectivity and a lower apparent activation energy, being a stable catalyst over long-time tests. DFT calculations confirm that C vacancies in V8C7 are responsible for the observed catalytic behavior, allowing reactants to adsorb more strongly and lowering the energy barrier for both H2 and CO2 dissociation steps. The present work highlights the importance of such native point defects in the transition metal carbides surface chemistry and catalytic properties.

Characterization of defect states in Mg-doped GaN-on-Si p+n diodes using deep-level transient Fourier spectroscopy

Mg-doped GaN-on-Si p+n diodes have been fabricated and characterized by static electrical and deep-level transient Fourier spectroscopy (DLTFS) measurements. From static capacitance-voltage (C–V) and current–voltage (I–V) characteristics, we estimated the diffusion barrier of the p+n diode close to the GaN band gap at room temperature. The temperature dependence of the capacitance showed freeze-out effect of the Mg-dopants at 200 K. From DLTFS measurements for various reverse bias and pulse voltages, two peaks were found and are composed of different defect states. The first peak with two components was related to diffusion of Mg p-type dopants in the n-GaN and VN-related defects. The two components have activation energies close to 0.25 eV, from valence band and conduction band with a capture cross-section of ∼10−16 cm2. The second peak with two components showed temperature shifts with the pulse height indicating a band-like behavior. This peak was commonly attributed to deep acceptor CN-related defects with an activation energy of E V +0.88 eV and a capture cross-section of 10−13 cm2. A second acceptor level was found, with an activation energy of 0.70 eV and a capture cross-section of 10−15 cm2. This second component was previously attributed to native point defects in GaN.