Low Doping Levels Research Articles

Composition optimization, high-temperature stability, and thermal cycling performance of Sc-doped Gd2Zr2O7 thermal barrier coatings: Theoretical and experimental studies

Sc was doped into Gd2Zr2O7 for expanding the potential for thermal barrier coating (TBC) applications. The solid solution mechanism of Sc in the Gd2Zr2O7 lattice, and the mechanical and thermophysical properties of the doped Gd2Zr2O7 were systematically studied by the first-principles method, based on which the Sc doping content was optimized. Additionally, Sc-doped Gd2Zr2O7 TBCs with the optimized composition were prepared by air plasma spraying using YSZ as a bottom ceramic coating (Gd-Sc/YSZ TBCs), and their sintering behavior and thermal cycling performance were examined. Results revealed that at low Sc doping levels, Sc has a large tendency to occupy the lattice interstitial sites, and when the doping content is above 11.11 at%, Sc substituting for Gd in the lattice becomes dominant. Among the doped Gd2Zr2O7, the composition with 16.67 at% Sc content has the lowest Pugh’s indicator (G/B) and the highest Poisson ratio (σ) indicative of the highest toughness, and the decreasing trends of Debye temperature and thermal conductivity slow down at this composition. By considering the mechanical and thermophysical properties comprehensively, the Sc doping content was optimized to be 16.67 at%. The fabricated Gd-Sc coatings remain phase and structural stability after sintering at 1400 °C for 100 h. Gd-Sc/YSZ TBCs exhibit excellent thermal shock resistance, which is related to the good thermal match between Gd-Sc and YSZ coatings, and the buffering effect of the YSZ coating during thermal cycling. These results revealed that Sc-doped Gd2Zr2O7 has a high potential for TBC applications, especially for the composition with 16.67 at% Sc content.

Visualizing the evolution from Mott insulator to Anderson insulator in Ti-doped 1T-TaS2

The electronic evolution of doped Mott insulators has been extensively studied for decades in search of exotic physical phases. The proposed Mott insulator 1T-TaS2 provides an intriguing platform to study the electronic evolution via doping. Here we apply scanning tunneling microscopy (STM) to study the evolution in Ti-doped 1T-TaS2 at different doping levels. The doping Ti atom locally perturbs the electronic and spin state inside the doped star of David and induces a clover-shaped orbital texture at low-doping levels (x < 0.01). The insulator to metal transition occurs around a critical point x = 0.01, in which small metallic and large insulating domains coexist. The clover-shaped orbital texture emerges at a broader energy range, revealing a competition with the electron correlation. It transforms to a disorder-induced Anderson insulating behavior as doping increases. We directly visualize the trapped electrons in dI/dV conductance maps. The comprehensive study of the series of Ti-doped 1T-TaS2 deepens our understanding of the electronic state evolution in a doped strong-correlated system.

First-principles calculation of chalcogen-doped Sr2M2O7 (M=Nb and Ta) for visible light photocatalysis

Doped Sr2M2O7 (M ​= ​Nb and Ta) crystals with various chalcogen X (X ​= ​S, Se, and Te) concentrations have been studied by using first-principles calculations combined with hybrid functional (HSE06). Our results indicate that the more stable doping site of X generally induces the less distortion of the lattice. The calculated formation energies reveal that the doping becomes more difficult in the order S ​< ​Se ​< ​Te. Some localized X np states are located above the VBM of pure semiconductors, which results in the narrowing of band gaps. For Sr2Nb2O7, an increase of the X concentration has negative effect on the narrowing of band gap since the X np states moves towards the VBM of pristine semiconductor. As to Sr2Ta2O7, the higher doping level of X elements leads to the further narrowing of energy gap compared with the lower doping level. The formation of Sr2M2X7 (X ​= ​S and Se) by replacing all the O atoms in crystal structures by chalcogens extends the absorption edges to near-infrared region and loses the abilities to oxidize water. Our findings demonstrate that S-doped Sr2Nb2O7, Se-doped Sr2Nb2O7 and Se-doped Sr2Ta2O7 at a higher doping level, and Te-doped Sr2Ta2O7 at a lower doping level can catalyze overall splitting water reaction under visible light irradiation. Furthermore, they possess stronger capabilities of photo-oxidation and photo-reduction than pristine semiconductors. Therefore, these systems mentioned above are potential visible-light-driven photocatalytic materials, which are worthy of further experimental investigations.

Doping MAPbBr3 hybrid perovskites with CdSe/CdZnS quantum dots: from emissive thin films to hybrid single-photon sources.

We report the first doping of crystalline methyl-ammonium lead bromide perovskite (MAPbBr3) films with CdSe/CdZnS core/shell quantum dots (QDs), using a soft-chemistry approach that preserves their high quantum yield and other remarkable luminescence properties. Our approach produces MAPbBr3 films of around 100 nm thickness, doped at volume ratios between 0.01 and 1% with colloidal CdSe/CdZnS QDs whose organic ligands were exchanged with halide ions to allow for close contact between the QDs and the perovskite matrix. Ensemble photoluminescence (PL) measurements demonstrate the retained emission of the QDs after incorporation into the MAPbBr3 matrix. Photoluminescence excitation (PLE) spectra exhibit signatures of wavelength-dependent coupling between the CdSe/CdZnS QDs and the MAPbBr3 matrix, i.e., a transfer of charges from matrix to QD, which increases the QD luminescence by up to 150%, or from QD to matrix. Spatially-resolved PL experiments reveal a strong correlation between the positions of QDs and an enhancement of the PL signal of the matrix. Lifetime imaging of the doped films furthermore shows that the emission lifetime of MAPbBr3 is slower in the vicinity of QDs, which, in combination with the increased PL signal of the matrix, suggests that QDs can act as local nucleation seeds that improve the crystallinity of MAPbBr3, thus boosting its emission quantum yield. Luminescence antibunching measurements provide clear evidence of single-photon emission from individual QDs in perovskite. Finally, the analysis of blinking statistics indicates an improvement of the photostability of individual QDs in perovskite as compared to bare CdSe/CdZnS QDs. At high CdSe/CdZnS QD doping levels, this work thus opens a route to hybrid solar concentrators for visible-light harvesting and hybrid-based LEDs, while a low degree of doping could yield hybrid single-photon sources than can be embedded in field-effect devices for single-charge control, which would allow the construction of nanophotonic devices via low-cost solution-processing techniques as an alternative to solid-state quantum dots.

Effect of boron doping on luminescent properties of silicon--vacancy and germanium--vacancy color centers in diamond particles obtained by chemical vapor deposition

The effect of doping with boron on the luminescent properties of diamond particles synthesized by Hot Filament Chemical Vapor Deposition technique with color centers embedded during the growth process has been studied. It is shown that at low boron doping level, the photoluminescence intensity of a narrow zero-phonon line of the silicon-vacancy color center (738.2 nm) demonstrates a strong dependence on the concentration of boron atoms at the sites of the diamond lattice. The dependence of the intensity of a broad photoluminescence band in the wavelength range 520-800 nm on the concentration of boron atoms in the gas mixture in the range from 14 to 64000 ppm has been analyzed. The Raman scattering spectra of the obtained particles have been studied. At the concentration of boron atoms in the gas mixture up to 1540 ppm, the Raman scattering spectra of diamond particles practically do not change when the boron concentration is varied. At high boron doping level, the diamond band in the Raman spectra exhibit a spectral response typical of the Fano resonance. Keywords: diamond particles, color centers, boron doping, photoluminescence, chemical vapor deposition.

Single‐Solution Doping Enabling Dominant Integer Charge Transfer for Synergistically Improved Carrier Concentration and Mobility in Donor–Acceptor Polymers

AbstractChemical doping of donor–acceptor (D–A) polymers is essential for their usage in highly efficient optoelectronic devices. The crucial challenge remains to synergistically improve the carrier concentration and mobility of these polymers via a single solution doping method. Here, a D–A polymer, Pg32T‐OTz is designed and synthesized, containing a weak‐acceptor–strong‐donor backbone with nonpolar (alkoxy) and polar (ethylene glycol) side chains. Pure integer charge transfer (ICT) is shown to occur in 2,3,5,6‐tetrafluoro‐tetracyanoquinodimethane (F4TCNQ)‐doped D–A polymer at a low doping level. It is shown that the ordered edge‐on orientations of ICT structures overcome the Coulomb interactions and endow a long‐range hole delocalization, while few charge transfer complex states form in amorphous regions and bridge the crystalline ICT structures at high doping level, creating a network that sustains efficient charge transport. As a result, simultaneously high carrier concentration and high mobility are realized for F4TCNQ‐doped Pg32T‐OTz and thus a high electrical conductivity up to 550 S cm−1 is approached, which is the highest value among any doped polymers via a single‐solution doping process. This work demonstrates that the modulation of the acceptor strength combined with side‐chain engineering is an effective molecular design strategy to promote both the doping efficiency and carrier transport property of D–A polymers.

Coupling behavior of Bi2Sr2−xInxCaCu2O8+d

The interplay between intrinsic grain and inter-grain effects at the superconducting state defines the behavior of bulk high-temperature superconductors. In this work, we use DC resistivity and AC magnetic susceptibility measurements to show how the intrinsic and inter-grain properties influence the intergrain phase coherence and flux dynamics of Bi2Sr2−xInxCaCu2O8+d. High In-doped samples, x ≥ 0.4, have broad resistive and diamagnetic superconducting transitions due to weak intergrain coupling caused by segregated impurities. Low doping levels, x ≤ 0.3, have sharp superconducting resistive and diamagnetic transitions, with higher transition temperature TC attributed to an increase superconducting pair density. This results to enhanced intergrain coupling and strong diamagnetic screening of intergrain void networks. An applied AC field of 0.77 mT amplitude deteriorates the intergrain diamagnetic screening at x = 0.2 through AC flux slip between grains.

ФОТОЕЛЕКТРИЧНИЙ МЕТОД ДІАГНОСТИКИ ПЕРЕХІДНИХ ШАРІВ НАПІВПРОВІДНИКОВИХ ГЕТЕРОСИСТЕМ

The diagnostic method is as follows: the lateral photo-EMF spectral characteristics are measured, generated in the structure (or device) when illuminated by wavelength light with a near the edge of the basic semiconductor layer. For illustrations of efficiency method the given part of the measurement results for Schottky contact samples with a nitrogen concentration of 5% and a thermal annealing temperature of 900 and 950°C. It has been found that a significant character and a small amplitude of such a characteristic indicates qualitative at a homogeneity and the necessary magnitude of the potential barrier (or barriers), that it is necessary to form to make Schottky contact or other structure. A significant characteristic and a small amplitude of such a characteristic indicates a qualitative one-line and the required value of a potential barrier (or barriers) that must be formed for the manufacture of a semiconductor structure or device. If the spectral characteristic has one maximum and amplitude that is many times higher than the amplitude of a significant characteristic, then this indicates a formed transition layer between components of heterosystems with high, compared with a quasine-power region of semiconductor, conductivity. The presence of such a layer increases the probability breaks down of the microelectronic device. Investigation of the distribution of lateral photours along the metal semiconductor interface compliant interpretation of spectral characteristics features. The linear significant form of distribution of EMF confirms the presence of a transition layer with a lower doping level compared with GaAs. An important feature of the diagnostic method is its non-destructive character, as well as the possibility of applying to semiconductor or devices based on them, in which the photovoltaic effect may occur.

Cu-TiO2/SiO2 photocatalysts for concrete-based building materials: Self-cleaning and air de-pollution performance

As a consequence of human activities, today, we are exposed to high pollution levels that cause building materials damage and many diseases. The application of TiO2 as coating on building materials has proven the efficiency of self-cleaning and air de-pollution. However, its performance is limited due to its absorption being exclusively located at UV light. In this study, Cu-TiO2 photocatalysts were integrated into silica sol–gel synthesis to manufacture new long-lasting term coatings with self-cleaning and air purification properties for concrete-based building materials. The enhancement of TiO2 photoactivity by incorporating Cu into its network has been clearly demonstrated. The self-cleaning and air de-polluting efficiency were assessed using methylene blue (MB), soot and NO, respectively, indicating that the presence of Cu promotes the TiO2 performance. For low doping levels, by increasing the copper loading, the TiO2 photoactivity increased. The highest efficiency was obtained by the photocatalyst containing 5% copper, which shows maximum destruction of methylene blue and soot of 95% and 50% within 60 min and 168 h of irradiation, respectively. However, higher copper loading led to a decrease in its performance, reaching a total degradation of MB and soot of 90% and 45% after 60 min and 168 h of irradiation, respectively, for the sample containing 15% of copper loading.

Investigation of structure, morphology, photoluminescence, linear and third-order nonlinear optical properties of Sn1−xLaxO2 thin films for optical limiting applications

Transparent nano-crystalline thin films of Sn1−xLaxO2 (x = 0–0.1) were deposited on glass substrates using the cost-effective chemical spray pyrolysis method. The influence of La3+ concentration on the structural, morphological, linear and third-order nonlinear optical properties of the films were analysed. All the films possessed tetragonal crystalline structure, having a preferred (1 1 0) plane orientation irrespective of the doping content. Till a doping of 3 at%, the crystallite size showed a decreasing tendency. The fibrous morphology exhibited by the pure film got transformed into dense spherical microstructures at lower La doping levels. The films with lower doping content showed above 60% of transmittance in the visible wavelength region. The modification in the energy band gaps of the films were observed due to variations in the La concentration. The presence of various defects identified in the deposits by photoluminescence (PL) spectra analysis in the ultraviolet and visible region. Near white light emission was noticed in the films due to these defects. All the films exhibited third-order optical nonlinearity with an observed switching between reverse saturable absorption (RSA) and saturable absorption (SA). The films exhibited self-focussing and defocussing nature due to variation in the doping level. Sn0.94La0.06O2 film was identified as suitable for optical limiting application due to its least limiting threshold value. Hence, these spray pyrolyzed La doped films can be used in optical and nonlinear optical devices.

Constructing stable continuous proton transport channels by in-situ preparation of covalent triazine-based frameworks in phosphoric acid-doped polybenzimidazole for high-temperature proton exchange membranes

Phosphoric acid (PA)-doped high-temperature proton exchange membranes (HTPEMs) suffer from low efficiency of proton transport and severe PA leakage. Constructing a stable continuous proton transport channel may be a promising method to address the above issues. Herein, a stable proton transport channel was constructed for the first time by in-situ preparing covalent triazine-based frameworks (CTFs) in polybenzimidazole under a mild trifluoromethanesulfonic acid (TFA) catalysis condition. The membranes were prepared with CTFs loading from 10% to 40%, and the properties of the membranes were characterized carefully. The membrane containing 30% CTFs showed some attractive properties, such as high conductivity (74.8 mS cm−1) under low PA doping level (167.1%), low volume swelling (71.8%), and high PA retention ability (89.5%). Importantly, under the same PA doping level, the single fuel cell assembled with the composite membranes showed a higher peak power density (534.4 mW cm−2) than that of poly [2,2′-(p-oxydiphenylene)-5,5′-benzimidazole] (OPBI) (325.2 mW cm−2). The results prove that constructing a stable proton transport channel can improve the properties of the membranes and the prepared membranes can potentially be used as HTPEMs.

Optical transitions from hexavalent chromium in lithium-borate glasses

The melt-quenching technique was used to prepare a series of chromium-doped borate glasses with the composition xCr2O3–(70 − x) B2O3–30 Li2O–(x = 0, 0.1, 0.2, 0.3 and 0.5 mol %). The low-doping level here employed allowed to unambiguously identify well-defined near-edge Cr6+ optical transitions, and to precisely determine the optical band gap of the borate glass host. Additional Cr3+ transitions were observed in the visible regime, rendering a strong modulation of the glass color, from colorless to dark greenish, with Cr content. Both Cr6+ (after the charge transfer transformation into Cr5+) and Cr3+oxidation states and their variations with Cr doping were identified from electron spin resonance spectroscopy. All samples exhibit similar vibrational spectra dominated by BO3 and BO4 structural units, with the development of weak Cr6+ vibration with Cr doping. The present study provides structurally similar but optically active and tunable glass hosts suitable for various optical applications.

Structure and Spin-Glass Magnetism of the MnxNi2Zn11–x Pseudobinary γ-Brasses at Low Mn Contents

The pseudobinary MnxNi2Zn11–x γ-brass-type phases at low Mn dopant levels (x = 0.1–0.5) were investigated. Crystal structures were determined for the two loading compositions of x = 0.3 and 0.5. The structures were solved in the cubic space group of I43m and are described in close analogy to the Ni2Zn11 parent γ-brass that is based on the 26-atom cluster, consisting of inner tetrahedron (IT), outer tetrahedron (OT), octahedron (OH), and cuboctahedron (CO). The refined site occupancies of the MnxNi2Zn11–x (x = 0.3, 0.5) reveal that the cluster center, which is empty in the Ni2Zn11, shows a partial occupation by Zn, with a partial depletion of Zn at the IT sites. The OH sites show a mixed Zn/Mn occupation. The OT and CO sites remain intact with respect to Ni2Zn11. Magnetic properties were studied for the Mn0.3Ni2Zn10.7 composition. The temperature-dependent zero-field-cooled and field-cooled magnetization, the ac susceptibility, the M(H) hysteresis curves, the thermoremanent magnetization, and the memory effect demonstrate typical broken-ergodicity phenomena of a magnetically frustrated spin system below the spin freezing temperature Tf ≈ 16 K. The Mn0.3Ni2Zn10.7 γ-brass phase classifies as a spin glass, originating predominantly from the random distribution of diluted Mn moments on the octahedral partial structure.

Impact of growth parameters on the background doping of GaN films grown by ammonia and plasma-assisted molecular beam epitaxy for high-voltage vertical power switches

We report on ammonia and plasma-assisted molecular beam epitaxy (NH3-MBE and PAMBE) grown GaN layers with a low net carrier concentration (Nnet). Growth parameters, such as growth rate, V–III ratio, and plasma power, were investigated on different substrates to study their impact on surface morphology and background doping levels using atomic force microscopy and capacitance–voltage (C–V) measurements, respectively. The elevated growth rates are especially interesting for vertical power switches, requiring very thick drift regions (over 10 μm) with low background concentrations. For our NH3-MBE-grown layers, Nnet shows an almost linear increase with the growth rate. Using a freestanding substrate and at a fast growth rate of 1.4 μm hr−1, a Nnet value as low as 1 × 1015 cm−3 was achieved. For samples grown via PAMBE, the lowest Nnet among samples grown under a Ga adlayer was 2 × 1016 cm−3 for a growth rate of 0.32 μm h−1 on a GaN-on-sapphire template. The results support the use of MBE for growing high-quality GaN material with reasonably fast growth rates maintaining low background doping levels for high-voltage vertical power electronic devices.

Synthesis and Characterisation of P-Doped Sr2MnFeO6-δ Systems

Perovskite systems have attracted considerable interest for fuel cell/electrolyser applications. Here we show that phosphate can be incorporated into the mixed Mn/Fe system Sr2MnFeO6-δ. In all cases, the conductivity was shown to increase for low doping levels (≤10 % phosphate incorporation) with a subsequent decrease for higher doping levels, attributed to the presence of Sr3(PO4)2 impurity as the solubility limit is exceeded. The work further illustrates that phosphate can be doped into perovskite systems, highlighting this doping approach as an alternative strategy to modify the structure and properties.

Realizing high thermoelectric performance in hot-pressed polycrystalline AlxSn1-xSe through band engineering tuning

SnSe-based thermoelectric materials are being explored since they have potential high thermoelectric figure of merit. We synthesized polycrystalline AlxSn1-xSe (x = 0.01, 0.02, 0.03 and 0.04) by hot-pressing method, and combined theoretical estimation with experimental measurement to investigate the influence of Al doping on thermoelectric properties of SnSe. It was found that dopant Al can effectively adjust the band structure of SnSe by introducing intermediate band. Al doping with low content (x = 0.01 and 0.02) can introduce a single intermediate band close to the valence band maximum or conduction band minimum, achieving band engineering optimization. In high temperature region (498 K < T < 823 K), the electronic transport properties significantly enhance with thermal excitation. The lattice thermal conductivity reduces with the Al atomic point defect scattering, leading to a low thermal conductivity of 0.47 W m−1K−1 in Al0.04Sn0.96Se at 823 K. As a result, a high ZT of 0.84 at 823 K is obtained from the Al0.04Sn0.96Se perpendicular to the pressing direction, which is 58.5% larger than that of SnSe. In addition, dopant Al can adjust the anisotropy of polycrystalline SnSe. The anisotropy of electronic properties are enhanced with low doping level (x = 0.01, 0.02) and suppressed with high doping level (x = 0.03, 0.04).

Role of oxygen vacancy defects in piezoelectric thermal stability characteristics of Mn-doped (K,Na,Li)NbO3 piezoceramics

Piezoelectric thermal stability characteristics of (K,Na,Li)NbO3 ceramics were investigated with a particular focus on the role of oxygen vacancy defects, by excluding the lattice distortion or microstructural effects with the help of a low level of Mn doping (less than 1.0 mol%). Benefiting from the reduction of intrinsic oxygen vacancy defects, controlled Mn doping (0.25 mol%) could achieve excellent ferroelectric/piezoelectric activity and superior thermal aging resistance. Beyond this doping level, however, the defect dipoles combined with extrinsic oxygen vacancies, although advantageous to the hardening effect, deteriorated the piezoelectric thermal stability and aging resistance including room-temperature properties. The thermal dissociation and migration of oxygen-vacancy-associated defect dipoles were considered mainly responsible for negating the ability to restore the initial poled domain state with defect-dipole-induced polarization. Results demonstrated the importance of keeping the intrinsic and extrinsic oxygen vacancy levels as low as possible for lead-free (K,Na)NbO3 systems with more thermally stable domain structure.

Highly Effective Self-Propagating Synthesis of Lamellar ZnO-Decorated MnO2 Nanocrystals with Improved Supercapacitive Performance.

A series of MOx (M = Co, Ni, Zn, Ce)-modified lamellar MnO2 electrode materials were controllably synthesized with a superfast self-propagating technology and their electrochemical practicability was evaluated using a three-electrode system. The results demonstrated that the specific capacitance varied with the heteroatom type as well as the doping level. The low ZnO doping level was more beneficial for improving electrical conductivity and structural stability, and Mn10Zn hybrid nanocrystals exhibited a high specific capacitance of 175.3 F·g−1 and capacitance retention of 96.9% after 2000 cycles at constant current of 0.2 A·g−1. Moreover, XRD, SEM, and XPS characterizations confirmed that a small part of the heteroatoms entered the framework to cause lattice distortion of MnO2, while the rest dispersed uniformly on the surface of the carrier to form an interfacial collaborative effect. All of them induced enhanced electrical conductivity and electrochemical properties. Thus, the current work provides an ultrafast route for development of high-performance pseudocapacitive energy storage nanomaterials.

Silicon-Based Quantum Mechanical Tunnel Junction for Plasmon Excitation from Low-Energy Electron Tunneling

Light emission from metal–insulator–semiconductor junctions (MISJs) has been explored for decades as a possible on-chip light source; however it is not clear whether the mechanism of light emission is plasmonic in nature or is dominated by electroluminescence. Previous studies only investigated silicon with low doping levels, but here we show that only highly doped silicon allows us to excite surface plasmon polaritons (SPPs) in MISJs via inelastic tunneling. This paper describes the mechanism of charge transport and light emission from silicon-based Au-SiO2-nSi MISJs as a function of the doping level Nd varying from 1.6 × 1015 cm–3 to 1.0 × 1020 cm–3. At low doping levels (Nd ∼ 1015 cm–3), the MISJs behave as Schottky diodes, and the mechanism of light emission involves a radiative recombination of electrons and holes from minority carrier injection under high applied bias (>5.5 V). With increasing doping levels, the current–voltage characteristics of the MISJs change, resulting in symmetrical current–voltage curves with parabolic conductance behavior characteristic of quantum mechanical tunneling. MISJs with the highest doping level (Nd ∼ 1020 cm–3) are dominated by quantum mechanical tunneling, and light emission originates from radiative decay of surface plasmon polaritons (SPPs) via scattering at threshold voltages as low as 1.5 V. Our simulations indicate that tunneling over the thin SiO2 barrier between the Au and highly doped nSi excites a hybrid-SPP mode localized to the Au whose dispersion depends on the effective index induced by the SiO2–nSi interface. Our studies show that Si needs to be sufficiently doped to be conductive enough to enable SPP excitation via inelastic tunneling.

Positive Seebeck Coefficient in Highly Doped La2−xSrxCuO4 (x = 0.33); Its Origin and Implication

We present a study of the thermoelectric (Seebeck and Nernst) response in heavily overdoped, non-superconducting La$_{1.67}$Sr$_{0.33}$CuO$_4$. In spite of the electron-like curvature of the Fermi surface, the Seebeck coefficient is positive at low temperatures. Such a feature, previously observed in copper, silver, gold and lithium, is caused by a non-trivial energy dependence of the scattering time. We argue that this feature implies a strong asymmetry between the lifetime of occupied and unoccupied states along the zone diagonals and such an electron-hole asymmetry impedes formation of Cooper pairs along the nodal direction in the superconducting ground state emerging at lower doping levels.