Shallow States Research Articles

Ultrafast Exciton Dynamics of CH3NH3PbBr3 Perovskite Nanoclusters.

Exciton dynamics of perovskite nanoclusters has been investigated for the first time using femtosecond transient absorption (TA) and time-resolved photoluminescence (TRPL) spectroscopy. The TA results show two photoinduced absorption signals at 420 and 461 nm and a photoinduced bleach (PB) signal at 448 nm. The analysis of the PB recovery kinetic decay and kinetic model uncovered multiple processes contributing to electron-hole recombination. The fast component (∼8 ps) is attributed to vibrational relaxation within the initial excited state, and the medium component (∼60 ps) is attributed to shallow carrier trapping. The slow component is attributed to deep carrier trapping from the initial conduction band edge (∼666 ps) and the shallow trap state (∼40 ps). The TRPL reveals longer time dynamics, with modeled lifetimes of 6.6 and 93 ns attributed to recombination through the deep trap state and direct band edge recombination, respectively. The significant role of exciton trapping processes in the dynamics indicates that these highly confined nanoclusters have defect-rich surfaces.

Timescale correlation of shallow trap states increases electrochemiluminescence efficiency in carbon nitrides

Highly efficient interconversion of different types of energy plays a crucial role in both science and technology. Among them, electrochemiluminescence, an emission of light excited by electrochemical reactions, has drawn attention as a powerful tool for bioassays. Nonetheless, the large differences in timescale among diverse charge-transfer pathways from picoseconds to seconds significantly limit the electrochemiluminescence efficiency and hamper their broad applications. Here, we report a timescale coordination strategy to improve the electrochemiluminescence efficiency of carbon nitrides by engineering shallow electron trap states via Au-N bond functionalization. Quantitative electrochemiluminescence kinetics measurements and theoretic calculations jointly disclose that Au-N bonds endow shallow electron trap states, which coordinate the timescale of the fast electron transfer in the bulk emitter and the slow redox reaction of co-reagent at diffusion layers. The shallow electron trap states ultimately accelerate the rate and kinetics of emissive electron-hole recombination, setting a new cathodic electrochemiluminescence efficiency record of carbon nitrides, and empowering a visual electrochemiluminescence sensor for nitrite ion, a typical environmental contaminant, with superior detection range and limit.

Aerosol assisted-chemical vapour deposition of tetrahedrite copper antimony sulphide thin films: the effect of zinc(II) impurities on optical properties

Non-stoichiometric semiconducting copper antimony sulphide (CAS) thin films, both undoped and with Zn2+ ions incorporated, were deposited at 500, 550 and 600 °C onto glass substrates by aerosol-assisted chemical vapour deposition (AACVD) using metal diethyldithiocarbamate precursors. Data from powder X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, and scanning electron microscopy – energy dispersive X-ray spectroscopy suggest a correlation of composition of the non-stoichiometric sulphur-deficient tetrahedrite phase (cubic structure) microcrystalline CAS (Cu12Sb4S13) thin films with particle sizes ranging from 0.1 to 4 µm. For undoped thin films, visible optical absorption with a bandgap of ∼2.1 eV is likely associated with compositional variations involving intrinsic lattice defects, including shallow electronic states such as copper interstitials and vacancies of sulphur as deep-lying donors, and copper-antimony anti-sites and antimony vacancies as deep-lying acceptors. Additionally, tetrahedrite phase CAS (tCAS) thin films with Zn2+ ions incorporated (Znx-tCAS) exhibit tunable composition-driven electronic structure for small Zn2+ content influencing narrower bandgaps between 1.7 and 1.9 eV. The Raman data suggest that the phase purity is affected by small fractions of the famatinite (tetragonal) phase. Overall, the thin films display broad emission of fast dual radiative recombination, and an additional recombination pathway exhibited for Znx-tCAS associated with zinc-related point defects. Though further studies are required to explore in more detail the defect chemistry, these results show the utility of AACVD in tuning compositional and optical properties of undoped tCAS and Znx-tCAS thin films.

Analysis on the capacitance-voltage characteristics of metal-insulator-semiconductor capacitors based on thermally evaporated WOx on n- and p- type crystalline silicon

Tungsten oxide (WOx) thin films were deposited on n- and p-type crystalline silicon (c-Si) wafers by thermal evaporation technique at room temperature. The electrical characterization of Ag/WOx/n-Si and Ag/WOx/p-Si metal–insulator-semiconductor (MIS) capacitors were examined by the capacitance- and conductance-voltage measurements as a function of applied frequency in the 10 kHz 10 MHz range. The admittance properties of both MIS devices have been evaluated by analyzing the interfacial nature and bulk c-Si in accumulation, depletion, and inversion regimes. The capacitance responses of interface traps, bulk c-Si, depletion, and shallow states at the WOx/c-Si interface indicated the appearance of the capacitance peaks in depletion and inversion regimes for Ag/WOx/n-Si and Ag/WOx/p-Si devices. From C-V characterization, various parameters such as the depletion layer width, depletion capacitance, flat-band voltage, total charge density, and net ionized state density were calculated as a function of applied frequency. For Ag/WOx/n-Si device, the total charge density and barrier height values of 7.642 × 10−9 C and 1.119 eV at 1 MHz, respectively, were obtained. The same parameters were calculated to be 5.887 × 10−8 C and 1.107 eV for Ag/WOx/p-Si. The results demonstrated that interfacial nature of WOx film significantly governs the electronic characteristics of the c-Si based MIS devices.

Selenium-alloyed tellurium oxide for amorphous p-channel transistors.

Compared to polycrystalline semiconductors, amorphous semiconductors offer inherent cost-effective, simple and uniform manufacturing. Traditional amorphous hydrogenated Si falls short in electrical properties, necessitating the exploration of new materials. The creation of high-mobility amorphous n-type metal oxides, such as a-InGaZnO (ref. 1), and their integration into thin-film transistors (TFTs) have propelled advancements in modern large-area electronics and new-generation displays2-8. However, finding comparable p-type counterparts poses notable challenges, impeding the progress of complementary metal-oxide-semiconductor technology and integrated circuits9-11. Here we introduce a pioneering design strategy for amorphous p-type semiconductors, incorporating high-mobility tellurium within an amorphous tellurium suboxide matrix, and demonstrate its use in high-performance, stable p-channel TFTs and complementary circuits. Theoretical analysis unveils a delocalized valence band from tellurium 5p bands with shallow acceptor states, enabling excess hole doping and transport. Selenium alloying suppresses hole concentrations and facilitates the p-orbital connectivity, realizing high-performance p-channel TFTs with an average field-effect hole mobility of around 15 cm2 V-1 s-1 and on/off current ratios of 106-107, along with wafer-scale uniformity and long-term stabilities under bias stress and ambient ageing. This study represents a crucial stride towards establishing commercially viable amorphous p-channel TFT technology and complementary electronics in a low-cost and industry-compatible manner.

Shallow Bound States and Hints for Broad Resonances with Quark Content b[over ¯]c[over ¯]ud in B-D[over ¯] and B^{*}-D[over ¯] Scattering from Lattice QCD.

We present the first determination of the energy dependence of the B-D[over ¯] and B^{*}-D[over ¯] isospin-0, S-wave scattering amplitudes both below and above the thresholds using lattice QCD, which allows us to investigate rigorously whether mixed bottom-charm b[over ¯]c[over ¯]ud tetraquarks exist as bound states or resonances. The scattering phase shifts are obtained using Lüscher's method from the energy spectra in two different volumes. To ensure that no relevant energy level is missed, we use large, symmetric 7×7 and 8×8 correlation matrices that include, at both source and sink, B^{(*)}-D[over ¯] scattering operators with the lowest three or four possible back-to-back momenta in addition to local b[over ¯]c[over ¯]ud operators. We fit the energy dependence of the extracted scattering phase shifts using effective-range expansions. We observe sharp peaks in the B^{(*)}-D[over ¯] scattering rates close to the thresholds, which are associated with shallow bound states, either genuine or virtual, a few MeV or less below the B^{(*)}-D[over ¯] thresholds. In addition, we find hints for resonances with masses of order 100MeV above the thresholds and decay widths of order 200MeV.

Synthesis and Optical Properties of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> Nanorods with Defects States

In this work, methylammonium lead halide CH3NH3PbI3 (MAPI3) perovskite nanorods were synthesized by the sol gel-spray ultrasonic method. The XRD pattern exhibits peaks assigned to the tetragonal MAPI3 structure with a residual PbI2 phase. SEM and AFM images show the formation of nanorods like picture with rough surface. The Raman spectrum was visualized to show various vibration modes in the film. Photoluminescence data revealed one emission peak at 786 nm (1.58 eV) in the band gap band, which was confirmed by the UV-Vis spectrum. The slight difference between the found band gap energy and the ideal one is explained in terms of shallow trap states.

Virtual and bound states in low-energy positron scattering by atoms and molecules via modified effective range theory

Modified effective range theory is applied as a tool to determine bound and virtual state energies in low-energy positron elastic scattering by atoms and molecules. This is achieved by the S-matrix continuation into the complex momentum plane, allowing to identify poles related to shallow energy states. The influence of the long-range polarization potential (∼r −4) on the bound and virtual-state pole positions is analyzed for noble gases and nonpolar molecules such as H2, N2, and CH4. The quantitative relations between the S-matrix poles and the s-wave scattering length accounting for dipole polarization are introduced.

Hidden-Charm Pentaquarks with Strangeness in a Chiral Quark Model

The LHCb collaboration has recently announced the discovery of two hidden-charm pentaquark states with strange quark content, Pcs(4338) and Pcs(4459); its analysis points towards having both hadrons’ isospins equal to zero and spin-parity quantum numbers 12− and 32−, respectively. Herein, we perform a systematical investigation of the qqscc¯(q=u,d) system by means of a chiral quark model, along with a highly accurate computational method, the Gaussian expansion approach combined with the complex scaling technique. baryon-meson configurations in both singlet- and hidden-color channels are considered. The Pcs(4338) and Pcs(4459) signals can be well identified as molecular bound states with dominant components ΛJ/ψ(60%) and ΞcD(23%) for the lowest-energy case and ΞcD∗(72%) for the highest-energy one. In addition, it seems that some narrow resonances can also be found in each allowed I(JP) channel in the energy region of 4.6–5.5 GeV, except for the 1(12−) channel where a shallow bound state with dominant Ξc∗D∗ structure is obtained at 4673 MeV with binding energy EB=−3 MeV. These exotic states are expected to be confirmed in future high-energy experiments.

Unlocking Invisible Defects of ZnSe Alloy Shells in Giant Quantum Dots with Near Unity Quantum Yield

AbstractPhotoluminescence quantum yield (PL QY) of colloidal quantum dots (QDs) can be improved by growing a shell, but it is rather limited if the shell thickness exceeds a threshold. Lattice mismatch between the core and shell is known to determine this critical shell thickness, securing QDs from defect formation through strain release. However, it cannot explain the recently reported high efficiency QDs with giant shells. Based on CdSe/ZnSe thick shell QDs, this study aims to identify the culprit limiting PL QY. In the shell growth process, the gradual reduction in PL QY is accompanied by a low‐energy tail emission, but the additional compressive strain by the outmost shell eliminates such abnormalities. It is revealed that the zinc vacancy in the shell provides shallow hole trap states. The computational study successfully explains the hole‐accepting zinc vacancy states above the CdSe 1Shh state, raised by compressive strain along radial direction. Additional hydrostatic compressive strain lifts the 1Shh state for this strained heterostructure to minimize the energetic gap with the zinc vacancy states. The finding suggests that critical shell thickness can be limited by atomic vacancy incorporated during shell growth, not by formation of misfit dislocation caused by strain release.

Overcoming Defect Limitations in Photocatalysis: Boron‐Incorporation Engineered Crystalline Red Phosphorus for Enhanced Hydrogen Production

AbstractPhotocatalytic hydrogen evolution (PHE) from water splitting is a promising technology for clean and renewable energy production. Elemental crystalline red phosphorus (CRP) is purposefully designed and developed for PHE reaction. However, the photocatalytic activity of CRP is limited by its intrinsic P vacancy (VP) defects, which lead to detrimental charge trapping at deep states and hence its severe recombination. To address this issue, a boron (B) incorporated CRP (B‐CRP) photocatalyst is tailored, synthesized via a simple and mild boric acid‐assisted hydrothermal strategy. The incorporation of B effectively fills the VP defects, reducing deep trap states (DTS) and introducing beneficial shallow trap states (STS) within the band structure of CRP. This defect engineering approach leads to enhanced photocatalytic activity, with B‐CRP achieving a PHE rate of 1392 µmol g−1 h−1, significantly outperforming most reported elemental photocatalysts in the literature. Density functional theory (DFT) simulations and ultrafast spectroscopy support the constructive role of B‐dopant‐induced STS in prolonging active charge carrier lifetimes, promoting more efficient photocatalytic reactions. The findings not only demonstrate the effectiveness of B‐CRP as a photocatalyst but also highlight the usefulness of dopant‐induced STS in advancing PHE technologies.

Epitaxial Growth of MgGa2O4/CsPbBr3 Heterostructure with Controlled Defect Tolerance for Dynamic Photoluminescence

AbstractEpitaxial growth of halide perovskite heterostructures endows unique electronic structures for diversified luminescent properties. Herein, an epitaxial growth strategy is proposed to assemble MgGa2O4/CsPbBr3 heterostructure with dynamic photoluminescence. Interface defect passivation during integration transforms interstitial and antisite interface defects of CsPbBr3 nanocrystals to shallow states, enhancing photoluminescence stability of the heterostructure against irradiation, solvents, and heat as well. Finally, the as‐constructed heterostructure shows a reversible dynamic photoluminescence from green to blue under 254 nm ultraviolet (UV) irradiation, which is attributed to the electron injection from shallow defects to the emission centers in MgGa2O4 and CsPbBr3 with different radiative recombination rates. Encryption applying binary encoding method based on MgGa2O4/CsPbBr3 and dynamic anti‐counterfeiting feature of its polymer hybrid film are further investigated.

Quantifying Dielectric Material Charge Trapping and De-Trapping Ability Via Ultra-Fast Charge Self-Injection Technique.

Recently, utilizing the air breakdown effect in the charge excitation strategy proves as an efficient charge injection technique to increase the surface charge density of dielectric polymers for triboelectric nanogenerators (TENGs). However, quantitative characterization of the ability of dielectric polymers to trap reverse charges and the effect on the startup time of secondary self-charge excitation (SSCE) are essential for extensive applications. Here, an ultra-fast charge self-injection technique based on a self-charge excitation strategy is proposed, and a standard method to quantify the charge trapping and de-trapping abilities of 23 traditional tribo-materials is introduced. Further, the relationship among the distribution of dielectric intrinsic deep, shallow trap states, and transportation of trapped charges is systematically analyzed in this article. It shows that the de-trapping rate of charges directly determines the reactivation and failure of SSCE. Last, independent of TENG contact efficiency, an ultra-high charge density of 2.67 mC m-2 and an ultra-fast startup time of SSCE are obtained using a 15µm poly(vinylidene fluoride-trifluoroethylene) film, breaking the historical record for material modification. As a standard for material selection, this work quantifies the charge trapping and de-trapping ability of the triboelectric dielectric series and provides insights for understanding the charge transport in dielectrics.

Robust Free Excitons in CH3NH3PbI3Halide Perovskite Revealed by Four‐Wave Mixing

AbstractDirect bandgap semiconductors possess a unique attribute: the presence of a free exciton state with a high total oscillator strength. This property makes them highly promising for applications in ultrafast optical signal processing and optical computing. One such protocol for optical computing is based on four‐wave mixing (FWM). In this study, the nonlinear optical effect in polycrystalline thin films of halide perovskite MAPbI3(MA+ = CH3NH) at low temperatures is demonstrated. Through analyzing the spectroscopy of the FWM signal, studying the photoluminescence excitation spectra, and comparing the findings with results from MAPbI3single crystals, it has been discovered that the strongest nonlinear response is observed at the free exciton resonance and in the region of shallow defect states. Surprisingly, the presence of FWM in cross‐linear excitation geometry has been observed, indicating potential involvement of other nonlinear or many‐body effects. The observations of FWM with free excitons even in highly defective MAPbI3thin films demonstrate the robustness of the exciton resonance and highlight the practical prospects for utilizing this material in optical computing.

Electronic Properties of F, Cl and H Atoms Adsorbed on CsPbI3(110) Surface

The electronic properties of the CsPbI3 surface are calculated by using first-principles calculations. Through calculating adsorption energy of CsPbI3 passivated by Cl, F and H atoms, it is found that adsorption energy values of CsPbI3 (110) surface passivated by H atoms fluctuate less, they are less sensitive to the adsorption position, and the adsorption is stable. Through calculating surface states of passivation system, it is found that F and H atoms have quite good passivation effect, and the Fermi level of passivation system increases and enters the forbidden band, indicating that the passivation effect is relatively ideal. Passivation effect of Cl atoms are poor, a new shallow acceptor defect states appeared at the bottom of the conduction band. This defect level could trap carriers and weaken the carrier migration ability, which is a disadvantage of Cl passivation system. Through the charge density difference and Bader charges, it is found that the F and H atoms get more electrons from the I atom, which is beneficial to the passivation of the surface states.

Interfacial Charge Transfer Bridge Prolongs Carrier Recombination Lifetimes of CoFe Metal‐Thiolate Framework/Hematite Photoanode for Water Oxidation

AbstractConstructing heterostructural photoanodes is attractive for elevating the photoelectrochemical (PEC) performance, however, it is a long‐standing challenge to achieve highly efficient interfacial charge transfer. Herein, a CoFe metal‐thiolate framework (CoFe MTF)/Fe2O3 photoanode connected by an interfacial Fe─O─N/S bond is designed to modulate the behavior of charge carriers and improve water oxidation performance. It is disclosed that this interfacial bond functions as a direct charge transfer bridge between shallow trap states of Fe2O3 and CoFe MTF, leading to prolonged carrier recombination lifetimes (85 ns for CoFe MTF/Fe2O3 compared to 37 ns for Fe2O3) and enhanced charge transfer efficiency. Alternatively, a robust interfacial electric field is established in the CoFe MTF/Fe2O3 p–n heterojunction, facilitating efficient charge transfer. As expected, the CoFe MTF/Fe2O3 photoanode exhibits significant enhancement in water oxidation, resulting in a three‐fold increase in photocurrent density compared to pristine Fe2O3. This study highlights the significance of designing interfacially bonded heterostructural photoelectrodes to regulate the transfer characters of charge carriers.

Infrared Irradiation‐Lattice Vibration Coupling‐Initiated N→Π* Electronic Transition in Carbon Nitride Nanosheets for Increased Photocatalysis

AbstractEnabling n→π* electronic transition in graphitic carbon nitride (g‐CN) can significantly enhance its photocatalytic performance by extending visible light absorption. However, achieving this transition remains a grand challenge due to the selection rule that restricts it to planar tri‐s‐triazine units of g‐CN. Here, an effective strategy to induce the n→π* transition in g‐CN nanosheets by coupling infrared irradiation with lattice vibrations is presented. Theoretical simulations reveal two prominent vibration modes perpendicular to tri‐s‐triazine units at 219 and 737 cm−1 can promote the corrugation of tri‐s‐triazine units and facilitate the n→π* electronic transition when coupled with infrared irradiation. Additionally, these vibrational modes enable the efficient exfoliation of bulk g‐CN into 2D nanosheets by overcoming the interplanar binding energy. Through femtosecond transient absorption spectra (fs‐TAS), the n→π* transition prolongs the lifetime of photocatalytically active shallow electron trapping states and charge separation states in g‐CN is demonstrated. Accordingly, the activated n→π* electronic transition and prolonged lifetime of photocatalytically active charges lead to an H2 generation rate of 2485 µmol h−1 gcat−1, ≈40 times higher than that of the bulk counterpart. This work highlights the effectiveness of coupling lattice vibrations with infrared irradiation to drive the n→π* transition for enhanced visible‐light photocatalysis.

Detrapping of the carriers from shallow states in a highly responsive, fast, broadband (UV-vis-NIR), self-powered SnSe/Si photodetector with asymmetric metal electrodes

Bias-dependent trapping and de-trapping can be seen in the SnSe device. At low bias, voltage trapping of the carriers can be seen in the device; they start to de-trap after 500 mV bias.

Defect structure-electronic property correlations in transition metal dichalcogenide grain boundaries.

Grain boundaries (Gb) in transition metal dichalcogenides are a rich source of interesting physics as well as a cause of concern because of its impact on electron transport across them in large area electronic device applications. Here, using first principles calculations, we show that beyond the conventional definition of grain boundaries based on misorientation, the defect structure present at the grain boundaries plays a significant role in defining the local electronic properties. We observed that even the standard 5-7 defect ring has differing electronic characteristics depending on its internal configuration. While the 5-7 ring presents shallow defect states, and induce long range strain fields with the local bandgap increasing up to 32.7%, the other commonly observed 4-8 defect rings introduce only mid-gap states, induce smaller strain fields with no observable bandgap change. The results show the seminal character of the individual defect structures at grain boundaries, and that their relative density can be used to determine the overall physico-chemical properties of the grain boundary.

Investigation on acceptor-donor co-doped SnO2 nanoparticles enriched with oxygen vacancies: a capacitive humidity sensor for respiration detection.

In this work, we develop a novel capacitive humidity sensor based on Al-Si acceptor-donor co-doped SnO2 for real-time monitoring of ambient humidity and human respiration. XRD measurements reveal that all samples exhibit a tetragonal rutile phase and the crystallite size of SnO2 decreases with increasing Al-Si content. The high intensity of the Raman peak at 762 cm-1 confirms the presence of bridging mode oxygen vacancies in (Al + Si)0.02Sn0.98O2. The EPR results show that the amount of singly ionized oxygen vacancies increases after the introduction of Al-Si. Both types and amounts of oxygen vacancy defects are particularly sensitive to the adsorption of water molecules. Moreover, according to DFT calculations, the contribution of the Si 3s orbital and Al 3s orbital to the band edge verifies the formation of acceptor-donor complexes in Al-Si co-doped SnO2. The humidity sensing results reveal that the (Al + Si)0.02Sn0.98O2 humidity sensor shows high sensitivity (S = 839), low hysteresis (1.94%) and fast response/recovery times (25 s/5 s). The respiratory intervals during shallow, medium and deep breathing states of (Al + Si)0.02Sn0.98O2 were measured at 2.8 s, 3.8 s and 4.5 s, respectively. The chemical mechanism for the enhancement of humidity sensing performance corresponding to the oxygen vacancy defects induced by Al-Si interplay is proposed.