Strong Built-in Electric Field Research Articles

High performance near infrared photodetector based on in-plane black phosphorus p-n homojunction

Black phosphorus (BP) has been considered as an appealing candidate for broadband photodetection from the visible to the infrared part of the spectrum owing to its tunable direct band gap and high carrier mobility. However, compared with other typical materials, the photoresponsivity of BP devices is rather weak. In this work, a near infrared photodetector with high performance based on in-plane BP p-n homojunction was investigated. The strong built-in electric field of the p-n junction can be modulated through bias and gate voltage, where the photo generated electron-hole pairs can be effectively separated, thus, leads to the enhanced photocurrent and faster photoresponse. Compared with the pristine BP, the photoresponsivity of the BP p-n homojunction under 1550 nm displays a 50-fold increment. The response time of the in-plane BP p-n homojunction photodetector exhibits two orders of magnitude improvement than that of the pristine BP and other lateral homogeneous BP p-n junction. Fascinatingly, the BP p-n photodetector also displays visible-infrared dual-band detection with well separated spectral response, opening up a feasibility of the intelligent identification of infrared targets.

Graphitic nanosheets via two-dimensional polymerization enhancing organic all-optically controlled photorefractive performance

All-optically controlled devices are the attractive subjects due to their future applications in all-optically controlled networks and computers. In this work, two-dimensional (2D) graphite nanosheets are synthesized to improve organic photorefractive performance in the all-optical operation, i.e., the photorefractive performance under zero electric field. The synthetic process is performed by 2D-polymerizing electrophilic and nucleophilic carbon species under solvothermal effect, where [:C≡C:]2− in calcium carbide is applied as nucleophilic carbon species and [C(Cl)] in carbon tetrachloride (CCl4) as electrophilic carbon species. The polycarbonization reaction consists of 1D horizontal growth for nanoribbon and 2D vertical growth for single-layer graphene. Then, single-layer graphene nanosheets are aggregated via π–π stacking effect into 2D graphite nanosheets being composed of multilayer graphene nanosheets. Notably, the self-template characteristics induce the synchronous and alternate ongoing of three above processes. Interestingly, when these 2D graphite nanosheets are doped into organic photorefractive devices, the all-optically controlled photorefractive performance is effectively improved by enhancing the gain coefficient of two-beam coupling about 46.4% from 80.1 cm−1 of control device to 117.2 cm−1 of nanographite-doped device. This is attributed to the excellent electric conductance of 2D graphite nanosheets, which strengthens the charge separation under the nonuniform light field inside organic photorefractive devices. Then, the stronger built-in electric field will be induced so that the orientation enhancement effect is increased in organic photorefractive devices. This work provides an effective and facile approach to improve organic all-optically controlled photorefractive performance with 2D graphitic nanosheets.

Na-Based monolayer photocatalysts with an extremely high intrinsic electric-field for water splitting.

A strong built-in electric field, high carrier mobility and a wide range of optical absorption values are the key parameters for photocatalysts used in water splitting. The design and preparation of photocatalysts possessing simultaneously these characteristics have always been the main tasks in the water splitting field. Here, we report a new family of 2D Na-based photocatalysts, NaAB2 (A = Al, Ga, In; B = S, Se, Te) monolayers, which may achieve this goal. First-principles calculations show that most of the NaAB2 monolayers are semiconductors with a suitable direct band gap ranging from visible to near-infrared light, exhibiting good optical absorption. The electron mobilities of the NaAB2 monolayers are up to 103 cm2 V-1 s-1, meaning the rapid migration of electrons can promote photocatalytic overall water splitting. Importantly, the electrostatic potential differences between the top surface and the bottom surface are larger than 1.23 eV for all the studied NaAB2 monolayers, meaning a high intrinsic built-in electric field that is present in these Na-based photocatalysts can promote the overall water splitting irrespective of their band gaps and band edges. Our studies show that the NaAB2 monolayers may be ideal photocatalysts for use in water splitting and may initiate a new round of experimental studies.

Interband optical absorption in wurtzite GaN/InxGa1−xN/GaN spherical quantum dots with built-in electric field

Based on the principle of density matrix and the finite element method, the interband optical absorption between the electron and hole has been investigated in a wurtzite [Formula: see text] spherical core–shell quantum dot (CSQD) including a strong built-in electric field (BEF). We have studied the effects of the size and the ternary mixed crystal on the optical absorption coefficients (ACs) and refraction index changes (RICs). The results indicate that the absorption peaks of ACs and RICs decrease rapidly, and show a redshift with the increase of the component [Formula: see text]. It is also found that the absorption peaks of ACs and RICs reduce obviously and depend on the core radius and the well width. When the core radius increases, the positions of the maximum ACs and RICs show a blueshift. At the same time, it presents a redshift when the well width increases. Particularly, the influence of the well width is much stronger than the core radius in the wurtzite [Formula: see text] spherical CSQDs. We hope that these results could provide guidance on both theoretical and experimental study related to the optical properties of spherical CSQDs.

Photocatalytic reduction for graphene oxide by PbTiO3 with high polarizability and its electrocatalytic application in pyrrole detection

Quick recombination of photogenerated electrons and holes in photocatalytic process remains a huge challenge. And the routine efforts are concentrated on heterojunction, metal decoration and surface defect strategies. PbTiO3 as a typical perovskite ferroelectrics is with a strong built-inelectric field as self-junction caused by internal spontaneous polarization, facilitating the charge separation in the photocatalytic process. Here, under UV irradiation, L-shaped PbTiO3 with active (1 1 0) facet as a photocatalyst was applied to photo-reduce graphene oxide (GO), where a specific reduced graphene oxide (rGO)/PbTiO3 composite was synthesized in presence of isopropanol, a hole-trapping agent. According to the linear optical properties, the polarizability of PbTiO3 is calculated to 1.01 × 10−23 cm3 (2.68 times that of P25 (TiO2)), inducing the photo-excited charge separation by PbTiO3. Based on XPS characterization, a TiOC chemical bond is identified on the interface between rGO and PbTiO3. The response peak current for an electrochemical sensor based on rGO/PbTiO3 was proportional to the concentration of pyrrole (6.6 × 10−9–3.1 × 10−7 M, R2 = 0.999), and an extremely low limit reaches to 2.38 × 10−9 M. In addition, polypyrrole during the pyrrole detection was realized by the multi-cycle oxidation process. And also, the electrochemical detection has been successfully applied for the pyrrole quantification in real samples.

Nonlinear optical absorption in wurtzite ZnCdO quantum wells

Nonlinear optical properties of a Zn1−xCdxO/ZnO quantum well structure originating from the transitions between the subbands within its conduction band are studied in terms of its dimension, Cd concentration and doping level. The intersubband transition energies are calculated within the effective mass and envelope function approximations. The Schrödinger and Poisson equations are self-consistently solved by considering the free and the bound charge distributions. The many-body effects, such as depolarization and excitonic shifts, are taken into account. The results are represented as functions the Cd concentration, limited between 0.05 and 0.2, and the well width, changing from 10 to 50 Å. The calculations indicate an intersubband transition energy range from 40 (9.67) to 138 meV (33.4 THz) and a blue-shift in it, as a result of the many-body effects, up to 11 meV in case of a doping level of 1 × 1025 m−3. It is shown that the transition energy increases with the widening quantum well within the range of the parameters as a result of the strong built-in electric field. The nonlinear absorption lineshape is found be to asymmetric and has the peak positions depending on the light intensity and the doping level inside the quantum well.

Temperature-dependent ultraviolet Raman scattering and anomalous Raman phenomenon of AlGaN/GaN heterostructure.

Temperature-dependent ultraviolet (UV) Raman scattering from AlGaN/GaN heterostructure is investigated. Compared to the visible Raman spectrum, four new peaks at 600, 700, 780, and 840 cm-1 are observed in the UV Raman spectrum. The peak at 780 cm-1 is from the AlGaN A1(LO) mode. According to the calculated dispersion relations of the interface phonon modes in the AlGaN/GaN heterostructure, the peaks at 600 and 840 cm-1 correspond to interface phonon modes. Meanwhile, the peak at 700 cm-1 is attributed to the disorder-active mode near the 2DEG interface. Due to the near-resonant enhancement effect, the intensities of the GaN A1(LO) mode, interface phonon modes, disorder active mode and the AlGaN A1(LO) mode exhibit different temperature dependence. Furthermore, the frequencies of the interface phonon modes and the disorder active mode show anomalous temperature dependence, which can be attributed to the strong built-in electric field near the 2DEG interface.

Interfacial coupling effects in g-C3N4/SrTiO3 nanocomposites with enhanced H2 evolution under visible light irradiation

The g-C3N4/SrTiO3 nanocomposite is an important material in photocatalysis, but little attention has been paid to their interfacial interaction in photocatalytic reaction. Herein, we prepare the g-C3N4/SrTiO3 nanocomposites via a two-step mechanically milling and calcination process. The composite exhibited the highest H2 evolution activity superior to that of the pure g-C3N4 and SrTiO3 in the visible light. The results of UV–vis DRS, PL and photoelectrochemical measurements demonstrated that g-C3N4/SrTiO3 exhibited more visible light adsorption and faster photo-generated charge transfer. Furthermore, the interfacial electronic structures of g-C3N4/SrTiO3 nanocomposites were thoroughly characterized. According to the XPS and DFT results, with the help of a strong built-in electric field presenting in the g-C3N4/SrTiO3 interface, the photo-generated electrons flow to the SrTiO3 from g-C3N4, leading to the highly-efficient electron separation and more H2O molecules photo-reduction to H2. This work explicates the significant role of built-in electric field in H2 evolution on g-C3N4/SrTiO3 photocatalyst.

Theoretical progress of polarized interfaces in semiconductors

The manipulation of electronic structures of conventional semiconductors remains the key issue of modern semiconductor physics and devices. Compare to limited modulation of semiconductors by conventional gate technique, we theoretically demonstrate that, polarized interfaces can generate a strong built-in electric field (about 10 MV/cm) in both polar and non-polar semiconductors, and the polarized interfaces can tune the band gaps in a wide range (approximately 0—2 eV), and significantly enhances the Rashba spin-orbit coupling strength as well. In this paper, we introduce polarized interfaces in polar semiconductor InN and non-polar semiconductor Ge, and generate topological insulator phases by polarized interfaces. The polarized interface is compatible with conventional semiconductor fabrication techniques and shows interesting physics and potential optoelectronic applications.

A Full‐Spectrum Metal‐Free Porphyrin Supramolecular Photocatalyst for Dual Functions of Highly Efficient Hydrogen and Oxygen Evolution

A full-spectrum (300-700 nm) responsive porphyrin supramolecular photocatalyst with a theoretical solar spectrum efficiency of 44.4% is successfully constructed. For the first time, hydrogen and oxygen evolution (40.8 and 36.1 µmol g-1 h-1 ) is demonstrated by a porphyrin photocatalyst without the addition of any cocatalysts. The strong oxidizing performance also presents an efficient photodegradation activity that is more than ten times higher than that of g-C3 N4 for the photodegradation of phenol. The high photocatalytic reduction and oxidation activity arises from a strong built-in electric field due to molecular dipoles of electron-trapping groups and the nanocrystalline structure of the supramolecular photocatalyst. The appropriate band structure of the supramolecular photocatalyst adjusted via the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of the porphyrin gives rise to thermodynamic driving potential for H2 and O2 evolution under visible light irradiation. Controlling the energy band structure of photocatalysts via the ordered assembly of structure-designed organic molecules could provide a novel approach for the design of organic photocatalysts in energy and environmental applications.

InGaAs Diodes for Terahertz Sensing-Effect of Molecular Beam Epitaxy Growth Conditions.

InGaAs-based bow-tie diodes for the terahertz (THz) range are found to be well suited for development of compact THz imaging systems. To further optimize design for sensitive and broadband THz detection, one of the major challenges remains: to understand the noise origin, influence of growth conditions and role of defects for device operation. We present a detailed study of photoreflectance, low-frequency noise characteristics and THz sensitivity of InGaAs bow-tie diodes. The diodes are fabricated from InGaAs wafers grown by molecular beam epitaxy (MBE) on semi-insulating InP substrate under different technological conditions. Photoreflectance spectra indicated the presence of strong built-in electric fields reaching up to 49 kV/cm. It was demonstrated that the spectral density of voltage fluctuations at room temperature was found to be proportional to 1/f, while at lower temperatures, 77–200 K, Lorentzian-type spectra dominate due to random telegraph signals caused by individual capture defects. Furthermore, varying bias voltage, we considered optimal conditions for device room temperature operation in the THz range with respect to signal-to-noise ratio. The THz detectors grown with beam equivalent pressure In/Ga ratio equal to 2.04 exhibit the minimal level of the low-frequency noise, while InGaAs layers grown with beam equivalent pressure In/Ga ratio equal to 2.06 are found to be well suited for fabrication of room temperature bow-tie THz detectors enabling sensitivity of 13 V/W and noise equivalent power (NEP) of 200 pW/√Hz at 0.6 THz due to strong built-in electric field effects.

High Response, Self-Powered Photodetector Based on the Monolayer MoS2/P-Si Heterojunction with Asymmetric Electrodes.

Here, a self-powered photodetector based on the monolayer MoS2/P-Si heterojunction with asymmetric electrodes was fabricated. The MoS2/p-Si heterojunction photodetector with asymmetric electrodes offers the advantages over the conventional heterojunction photodetector on optoelectronic applications in terms of strong built-in electric field and fast photogenerated carrier separation and transport. Significantly, the MoS2/P-Si heterojunction exhibited an obvious photovoltaic effect, which can be used as the self-powered photodetector operating without any bias voltage. At a voltage bias of 0 V, the photocurrent of the detector is 23 nA, and its photoresponse/recovery time is 84 ms/136 ms. When at bias, the detector shows a ratio of photocurrent to dark current up to 3120, high responsivity of 117 A W-1, and fast photoresponse/recovery time of 74 ms/115 ms. Our work illustrates the great potential of the MoS2/P-Si heterojunction device with asymmetric electrodes on photovoltaic applications.

Investigation of intersubband transition optical absorption in Zn1−xMgxO/MgO/ZnO heterostructures

The intersubband transition optical absorption (ITOA) properties of Zn1−xMgxO/MgO/ZnO heterostructures are investigated by the density matrix method. To calculate the electronic structure parameters in the proposed quantum well structure, a numerical method of solving the Schrödinger equation and Poisson equation self-consistently is carried out with considering the impact of the built-in electric field (BEF) induced by spontaneous polarization and piezoelectric polarization. The results show that the strong BEF is on the order of MV/cm and it can be modulated effectively by the Mg compositions in the Zn1−xMgxO barrier layer. Moreover, with the increase of Mg compositions or MgO insertion layer thickness, the decrease of ZnO well layer or ZnMgO barrier layer thickness, the peaks of ITOA coefficients increase and show pronounced blue shifts. This work is helpful for the research and development of ZnMgO/MgO/ZnO heterostructure devices.

Carrier Dynamics Engineering for High-Performance Electron-Transport-Layer-free Perovskite Photovoltaics

Summary Electron-transport-layer-free (ETL-free) device architectures are promising designs for perovskite photovoltaics because they offer simpler configurations, low cost, and convenience for versatile optoelectronics. However, the development of ETL-free photovoltaics is hindered by their low performance. Herein, we reveal that a low electron-injection rate at the ETL-free interface is responsible for the performance loss. Moreover, we demonstrate that improving carrier lifetimes in the perovskite films can remedy the poor carrier extraction at interfaces, enabling carrier collection efficiency in ETL-free photovoltaics to approach that in ETL-containing devices. Using perovskite films with microsecond carrier lifetimes, we obtained ETL-free devices with a power conversion efficiency (PCE) of 19.5%, nearly eliminated hysteresis, and good stability. Such a PCE value is comparable to that (20.7%) of the analogous ETL-containing photovoltaics. These results offer opportunities for ETL-free architecture designs in the perovskite photovoltaics family. More importantly, this research provides a general approach to improving the performance of photovoltaics with low-injection-rate interfaces.

Degradation of KNN-Based Lead-Free Piezoelectric Material Under Gamma Irradiation

(Na0.5K0.5)NbO3-(Bi0.5Li0.5)TiO3-BaZrO3 lead-free piezoelectric ceramics are irradiated to 50 Mrad(Si) by Co-60 gamma rays. The degradation behaviors of piezoelectric and ferroelectric characteristics are investigated. Most characteristics degrade fast during the first 10-Mrad(Si) irradiation and then degrade slowly. The 30% degradation of material performance at about 15 Mrad(Si) has been observed, which is much worse than that of lead zirconate titanate [Pb(Zr52Ti48)O3, PZT] ceramics. The (K, Na)NbO3-based ceramics have much more complex microstructures and interfaces than PZT ceramics, leading to the existence of much more defects and neutral traps. During irradiation, more ionized carriers are trapped. Stronger built-in electric field is established by inducing faster degradation of the material performance. The saturation of trapped charges at the domain wall might be the cause of the slowly degradation curve. Therefore, to successfully replace the lead-based piezoelectric material and improve the radiation tolerance in extreme radiation environment, the appropriate material compositions and corresponding fabrication process need to be considered.

High-performance photodetector based on hybrid of MoS2 and reduced graphene oxide

2D materials are a promising new class of materials for next generation optoelectronic devices owing to their appealing optical and electrical properties. Pristine molybdenum disulfide (MoS2) is widely used in next generation photovoltaic and optoelectronic devices, but its low photo-dark current ratio prevents its use in highly efficient photo detection applications. Here, we decorated crumpled reduced graphene oxide (rGO) particles on a large-area vertically aligned MoS2 flake network to enhance the performance of the MoS2-based photodetector by forming multiple nanoscale p–n heterojunctions. The rGO/MoS2 device exhibited a significantly improved photoresponsivity of ∼2.10 A W−1 along with a good detectivity of ∼5 × 1011 Jones (Jones = cm Hz1/2/W) compared to that of the pristine MoS2 photodetector in ambient atmosphere. Moreover, the rGO/MoS2 photodetector showed a fast response of ∼18 ms with excellent stability and reproducibility in ambient air even after three months. The high performance of the photodetector is attributed to enhanced photoexcited carrier density and suppressed photo generated electron–hole recombination due to the strong local built-in electric field developed at the rGO/MoS2 interface. Our results showed that integration of rGO with MoS2 provides an efficient platform for photo detection applications.

Interfacial coupling induced direct Z-scheme water splitting in metal-free photocatalyst: C3N/g-C3N4 heterojunctions

Mimicking the natural photosynthesis in green plants, artificial Z-scheme photocatalysis enables more efficient utilization of solar energy for photocatalytic water splitting. Most currently designed g-C3N4-based Z-scheme heterojunctions are usually based on metal-containing semiconductor photocatalysts, thus exploiting metal-free photocatalysts for Z-scheme water splitting is of huge interest. Herein, we propose two metal-free C3N/g-C3N4 heterojunctions with the C3N monolayer covering g-C3N4 sheet (monolayer or bilayer) and systematically explore their electronic structures, charge distributions and photocatalytic properties by performing extensive hybrid density functional calculations. We clearly reveal that the relative strong built-in electric fields around their respective interface regions, caused by the charge transfer from C3N monolayer to g-C3N4 monolayer or bilayer, result in the bands bending, renders the transfer of photogenerated carriers in these two heterojunctions following the Z-scheme instead of the type-II pathway. Moreover, the photogenerated electrons and holes in these two C3N/g-C3N4 heterojunctions can not only be efficiently separated, but also have strong redox abilities for water oxidation and reduction. Compared with the isolated g-C3N4 sheets, the light absorption in visible to near-infrared region are significantly enhanced in these proposed heterojunctions. These theoretical findings suggest that these proposed metal-free C3N/g-C3N4 heterojunctions are promising direct Z-scheme photocatalysts for solar water splitting.

Ferroelectricity in Hf0.5Zr0.5O2 Thin Films: A Microscopic Study of the Polarization Switching Phenomenon and Field-Induced Phase Transformations.

Because of their full compatibility with the modern Si-based technology, the HfO2-based ferroelectric films have recently emerged as viable candidates for application in nonvolatile memory devices. However, despite significant efforts, the mechanism of the polarization switching in this material is still under debate. In this work, we elucidate the microscopic nature of the polarization switching process in functional Hf0.5Zr0.5O2-based ferroelectric capacitors during its operation. In particular, the static domain structure and its switching dynamics following the application of the external electric field have been monitored with the advanced piezoresponse force microscopy (PFM) technique providing a nm resolution. Separate domains with strong built-in electric field have been found. Piezoresponse mapping of pristine Hf0.5Zr0.5O2 films revealed the mixture of polar phase grains and regions with low piezoresponse as well as the continuum of polarization orientations in the grains of polar orthorhombic phase. PFM data combined with the structural analysis of pristine versus trained film by plan-view transmission electron microscopy both speak in support of a monoclinic-to-orthorhombic phase transition in ferroelectric Hf0.5Zr0.5O2 layer during the wake-up process under an electrical stress.

Elevated-temperature luminescence measurements to improve spatial resolution

Various branches of applied physics use luminescence based methods to investigate light-emitting specimens with high spatial resolution. A key problem is that luminescence signals lack all the advantages of high locality (i.e. of high spatial resolution) when structures with strong built-in electric field are measured. Such fields exist intentionally in most photonic structures, and occur unintentionally in many other materials. In this case, as a result of beam-induced current generation and its outflow, information that indicates irregularities, nonuniformities and inhomogeneities, such as defects, is lost. We show that to avoid nonlocality and enable truly local luminescence measurements, an elevated measurement temperature as high as 350 K (or even higher) is, perhaps surprisingly, advantageous. This is in contrast to a widely used approach, where cryogenic temperatures, or at least room temperature, are recommended. The elevated temperature of a specimen, together with the current outflow being limited by focused ion beam (FIB) milling, is shown to improve the spatial resolution of luminescence measurements greatly. All conclusions drawn using the example of cathodoluminescence are useful for other luminescence techniques.

Prediction of an extremely long exciton lifetime in a Janus-MoSTe monolayer.

The electron-hole separation efficiency is a key factor that determines the performance of two-dimensional (2D) transition metal dichalcogenides (TMDs) and devices. Therefore, searching for novel 2D TMD materials with a long timescale of carrier lifetime has become one of the most important topics. Here, based on time-domain density functional theory (TD-DFT), we propose a brand new TMD material, namely Janus-MoSTe, which exhibits a strong built-in electric field. Our results show that in the Janus-MoSTe monolayer, the exciton consisting of an electron and hole has a relatively wide spatial extension and low binding energy. In addition, a slow electron-hole recombination process is observed, with a timescale on the order of 1.31 ns, which is even comparable to those of van der Waals (vdW) heterostructures. Further analysis reveals that the extremely long timescale for electron-hole recombination could be ascribed to the strong Coulomb screening effect as well as the small overlap of wavefunctions between electrons and holes. Our findings establish the built-in electric field as an effective factor to control the electron-hole recombination dynamics in TMD monolayers and facilitate their future applications in light detection and harvesting.