Jiuxiang Dai

Abstract

Surface charge transfer doping has been widely utilized to tune the electronic and optical properties of semiconductor photodetectors based on low-dimensional materials. Although many studies have been conducted on the performance (response time, responsivity, etc.) of doped photodetectors and their mechanisms, they merely examined a specific thickness and did not systematically explore the dependence of doping effects on the number of layers. This work performs a series of investigations on ReS₂ photodetectors with different numbers of layers and demonstrates that the p-dopant tetrafluorotetracyanoquinodimethane (F₄-TCNQ) converts the deep trap states into recombination centers for few-layer ReS₂ and induces a vertical p-n junction for thicker ReS₂. A response time of 200 ms is observed in the decorated 2-layer ReS₂ photodetector, more than two orders of magnitude faster than the response of the pristine photodetector, due to the disappearance of deep trap states. A current rectification ratio of 30 in the F₄-TCNQ-decorated sandwiched ReS₂ device demonstrates the formation of a vertical p-n junction in a thicker ReS₂ device. The responsivity is as high as 2,000 A/W owing to the strong carrier separation of the p-n junction. Different thicknesses of ReS₂ enable switching of the prominent operating mechanism between transforming deep trap states into recombination centers and forming a vertical p-n junction. The thickness-dependent doping effect of a two-dimensional material serves as a new mechanism and provides a scheme toward improving the performance of other semiconductor devices, especially optical and electronic devices based on low-dimensional materials.

Abstract

Chemical vapor deposition (CVD)-grown graphene films on Cu foils, exhibiting fine scalability and high quality, are still suffering from the adverse impact of surface contamination, i.e., amorphous carbon. Despite the recent successful preparation of superclean graphene through Cu-vapor-assisted reactions, the formation mechanism of amorphous carbon remains unclear, especially with regard to the functions of substrates. Herein, we have found that the crystallographic orientations of underlying metal substrates would determine the cleanness of graphene in such a way that slower diffusion of active carbon species on as-formed graphene-Cu(100) surface is the key factor that suppresses the formation of contamination. The facile synthesis of clean graphene is achieved on the meter-sized Cu(100) that is transformed from the polycrystalline Cu foils. Furthermore, a clean surface of graphene on Cu(100) ensures the reduction of transfer-related polymer residues, and enhanced optical and electrical performance, which allows for versatile applications of graphene in biosensors, functioning as flexible transparent electrodes. This work would offer a promising material platform for the fundamental investigation and create new opportunities for the advanced applications of high-quality graphene films.

A time-, spin-, and angle-resolved photoemission study of the ferroelectric Rashba semiconductor α-GeTe is performed following ultrafast optical excitation. The thermalization pathways exhibit an unconventional temperature dependence, which is understood in terms of the underpinning microscopic scattering channels. The Rashba effect is found to inhibit electron–electron scatterings, enabling control of the overall dynamics with lattice temperature on subpicosecond timescales.

Abstract

A large Rashba effect is essential for future applications in spintronics. Particularly attractive is understanding and controlling nonequilibrium properties of ferroelectric Rashba semiconductors. Here, time- and angle-resolved photoemission is utilized to access the ultrafast dynamics of bulk and surface transient Rashba states after femtosecond optical excitation of GeTe. A complex thermalization pathway is observed, wherein three different timescales can be clearly distinguished: intraband thermalization, interband equilibration, and electronic cooling. These dynamics exhibit an unconventional temperature dependence: while the cooling phase speeds up with increasing sample temperature, the opposite happens for interband thermalization. It is demonstrated how, due to the Rashba effect, an interdependence of these timescales on the relative strength of both electron–electron and electron–phonon interactions is responsible for the counterintuitive temperature dependence, with spin-selection constrained interband electron–electron scatterings found both to dominate dynamics away from the Fermi level, and to weaken with increasing temperature. These findings are supported by theoretical calculations within the Boltzmann approach explicitly showing the opposite behavior of all relevant electron–electron and electron–phonon scattering channels with temperature, thus confirming the microscopic mechanism of the experimental findings. The present results are important for future applications of ferroelectric Rashba semiconductors and their excitations in ultrafast spintronics.

III-Nitride Optoelectronic Devices

In article number 2106757, Lixia Zhao and co-workers review the recent progress of III-nitride (III-N) optoelectronic devices, such as light emitting diodes, photodetectors, solar cells and light photocatalysis, integrated with optical resonances, including surface plasmons, distributed Bragg reflectors and micro cavities. They also give an insightful prospect for the development of future III-N based optoelectronic devices.

Nature Communications, Published online: 07 April 2022; doi:10.1038/s41467-022-29495-y

A polycrystalline MoS₂-based thin-film transistor (TFT) is fabricated using radio frequency (RF) sputtering deposition and a sulfurization process. The main carrier transport mechanism in the polycrystalline MoS₂ is demonstrated through low-temperature electrical characteristics. Understanding the grain-dependent carrier transport of the polycrystalline MoS₂ is a promising approach for optimizing and designing next-generation polycrystalline transition metal dichalcogenides (TMDs) devices.

Abstract

Molybdenum disulfide (MoS₂) synthesis methods have become diverse and enable wafer-scale growth for high-performance optoelectronic applications. However, there has been limited research on the carrier transports of wafer-scale deposited MoS₂ thin-film transistors (TFTs). In this paper, the first demonstration of the electron transport mechanism in top-gated polycrystalline crystalline MoS₂ (poly-MoS₂) TFTs grown by a wafer-scale deposition method is presented. The MoS₂ is synthesized via radio frequency (RF) magnetron sputtering and gas flow chemical vapor sulfurization. A surface analysis is performed to determine the basic ingredients and grain size of the grown MoS₂. Furthermore, the electrical properties and charge transport behaviors of the poly-MoS₂ TFTs are characterized using current–voltage measurement at low temperatures (93–273 K). The extracted parameters (e.g., field-effect mobility, contact and channel resistance, activation energy, and hopping distance) and 2D Mott variable range hopping (VRH) of the poly-MoS₂ TFTs support the notion that the primary mechanism of carrier transport in the poly-MoS₂ TFTs involves thermally active hopping and grain effects. For advanced poly-MoS₂-based devices, an increase of grain size will be the principal factor using the relationship between the grain size and electron hopping distance of poly-MoS₂.

A low-voltage memristor array based on ultrathin PdSeO _x /PdSe₂ heterostructure is demonstrated using controllable ultraviolet–ozone treatment. By confining the formation of conductive filaments in the heterostructure, the memristor achieves a remarkable uniform switching with low set and reset voltage variability of 4.8% and −3.6%, respectively. The crossbar array further enables multiple convolutional image processing with a high image-recognition accuracy of ≈93.4%.

Abstract

In-memory computing based on memristor arrays holds promise to address the speed and energy issues of the classical von Neumann computing system. However, the stochasticity of ions’ transport in conventional oxide-based memristors imposes severe intrinsic variability, which compromises learning accuracy and hinders the implementation of neural network hardware accelerators. Here, these challenges are addressed using a low-voltage memristor array based on an ultrathin PdSeO _x /PdSe₂ heterostructure switching medium realized by a controllable ultraviolet (UV)–ozone treatment. A distinctively different ions’ transport mechanism is revealed in the heterostructure that can confine the formation of conductive filaments, leading to a remarkable uniform switching with low set and reset voltage variability values of 4.8% and −3.6%, respectively. Moreover, convolutional image processing is further implemented using various crossbar kernels that achieve a high recognition accuracy of ≈93.4% due to the highly linear and symmetric analog weight update as well as multiple conductance states, manifesting its potential beyond von Neumann computing.

A series of air-stable 2D non-van der Waals Cr₂Te _x Se_{3− x} (x = 0–3) compounds are designed, which exhibit prominent composition-dependent magnetic ordering and electronic states. Moreover, they all have favorable energetic/thermodynamic/mechanical stability, superior oxidation resistance, tunable magnetic ground states and bandgaps, and high magnetic transition temperature. Among them, 2D Cr₂TeSe₂ shows a sizable anomalous Hall conductivity.

Abstract

Since the confirmation of long-range magnetic ordering in 2D materials, much effort has been devoted to finding more 2D magnetic materials with high Curie temperature (T _C) and tunable physical properties. Most of the 2D magnets realized to date are based on van der Waals (vdW) materials, but recently synthesized 2D non-vdW ultrathin materials have revealed an alternative direction for the discovery of novel 2D magnets. In this paper, a family of 2D non-vdW Cr₂Te _x Se_{3− x} compounds is proposed. All Cr₂Te _x Se_{3− x} compounds are predicted to be energetically favorable and air stable, suggesting experimental feasibility, and long-term stability. Compared to other non-vdW materials, Cr₂Te _x Se_{3− x} compounds have superior oxidation resistance. Moreover, they exhibit obvious composition-dependent magnetic ordering, which originates from the competition between interlayer direct antiferromagnetic exchange and ferromagnetic superexchange. Among them, half-metals Cr₂Te₃ and Cr₂TeSe₂ exhibit robust ferromagnetism with high T _C values of 396 and 261 K, respectively. Additionally, 2D Cr₂TeSe₂ shows a sizable anomalous Hall conductivity of 6237 Ω cm⁻¹. The present results enrich the database of 2D non-vdW magnets and provide fundamental understanding about the composition-induced magnetic phase transition, which may be of use in spintronic applications.

Nature Communications, Published online: 08 April 2022; doi:10.1038/s41467-022-29545-5

Droplet nucleation site density (N _s) declines exponentially and the distance between droplet centers increase linearly as wettability decreases. This nucleation behavior is in line with existing models for a Rayleigh droplet distribution. An exponential function is proposed to predict N _s with respect to contact angle in conditions relevant to atmospheric water capture.

Abstract

Dew water is recognized as a valuable source of clean water for human consumption. Given the developing concerns over global water scarcity, great focus has been turned toward increasing the efficiency of existing dew harvesting methodologies. Droplet nucleation is a critical first stage in the condensation process – and therefore key to dew water harvesting. In this paper, the droplet nucleation site density (N _s) as a function of surface wettability on smooth thin polymer films is quantified. A custom-built environment chamber, operated in low supersaturation conditions relevant to atmospheric water harvesting, allows strict experimental control over temperature and humidity. Droplet growth through coalescence is quantified, and an exponential increase in the rate of coalescence is seen as the test surface wettability increased. N _s declines exponentially as wettability decreases according to the fitted equation N _s = 1.1 × 10¹¹ e ^−0.043θ, where theta is the contact angle of the smooth surface, which can be used to predict N _s from a known contact angle, currently not available. The average distance between droplet centers is found to increase at a linear rate. This nucleation behavior is in line with those of droplets in a Rayleigh distribution.

The heterojunctions composing of p-type GeSe and n-type MoTe₂ exhibit broad spectral coverage up to 1310 nm, extremely low dark current of several tens of fA, large linear dynamic range of 118 dB, and considerable polarization sensitivity of 5.4. This work offers the concept of anisotropic/isotropic heterostructure toward self-powered, near-infrared, and polarization-sensitive photodetectors.

Abstract

Near-infrared polarization-sensitive photodetectors hold the advantages of capturing light signals with high performance while shielding stray light, endowing them the potential applications in target tracking, remote sensing, and computer vision. Here, a 2D polarization-sensitive, self-powered, and near-infrared photodetector is constructed by vertically stacking multilayer p-type GeSe on n-type MoTe₂. The type-II energy band alignment and anisotropic crystalline structure of GeSe components allow an effective separation and transmission of polarized light excited carriers, enabling the capability of polarization-sensitive and self-powered photodetections. The device exhibits broadband spectral coverage from visible (405 nm) to near-infrared (1310 nm) wavelength range. At zero bias and 808 nm light, the responsivity (R) and detectivity (D*) can reach 52 mA W^–1 and 4.1 × 10¹¹ Jones, respectively. Due to the extremely low dark current of several tens of fA, the photoswitching ratio can reach close to 10⁶. More importantly, because of the strong in-plane anisotropic orthogonal structure of GeSe, the polarization sensitivity can reach 5.4 under 635 nm polarized light illumination, outperforming the polarization-sensitive photodetectors based on 2D anisotropic materials and heterostructures. This work provides an effective strategy of using anisotropic/isotropic GeSe/MoTe₂ heterojunctions to realize self-powered, near-infrared, and polarization-sensitive photodetectors with integrated angle-resolved optoelectronic devices.

Noncollinear antiferromagnetic metals are hot spots in the magnetic community. The chemical potential is found to effectively switch on/off the anomalous Hall effect (AHE) of such metals, which opens a new path to tuning the AHE and experimentally validates the essential role of chemical potential in theoretical calculations for determining the anomalous Hall conductivity.

Abstract

The discovery of the anomalous Hall effect in noncollinear antiferromagnetic metals represents one of the most important breakthroughs for the emergent antiferromagnetic spintronics. The tuning of chemical potential has been an important theoretical approach to varying the anomalous Hall conductivity, but the direct experimental demonstration has been challenging owing to the large carrier density of metals. In this work, an ultrathin noncollinear antiferromagnetic Mn₃Ge film is fabricated and its carrier density is modulated by ionic liquid gating. Via a small voltage of ≈3 V, its carrier density is altered by ≈90% and, accordingly, the anomalous Hall effect is completely switched off. This work thus creates an attractive new way to steering the anomalous Hall effect in noncollinear antiferromagnets.

The fabrication of centimeter-scale, crack-free, freestanding Hf_0.5Zr_0.5O₂ nanomembranes is reported, and their robust ferroelectricity is confirmed. By transferring freestanding Hf_0.5Zr_0.5O₂ membranes to transmission electron microscopy grids, the phases, orientations, grain-size distributions, and grain boundaries are atomically characterized from plan-view. This work opens a new paradigm for exploring the complex structures and unconventional ferroelectricity in polymorphic Hf_0.5Zr_0.5O₂ using plan-view imaging.

Abstract

Hafnia-based compounds have considerable potential for use in nanoelectronics due to their compatibility with complementary metal–oxide–semiconductor devices and robust ferroelectricity at nanoscale sizes. However, the unexpected ferroelectricity in this class of compounds often remains elusive due to the polymorphic nature of hafnia, as well as the lack of suitable methods for the characterization of the mixed/complex phases in hafnia thin films. Herein, the preparation of centimeter-scale, crack-free, freestanding Hf_0.5Zr_0.5O₂ (HZO) nanomembranes that are well suited for investigating the local crystallographic phases, orientations, and grain boundaries at both the microscopic and mesoscopic scales is reported. Atomic-level imaging of the plan-view crystallographic patterns shows that more than 80% of the grains are the ferroelectric orthorhombic phase, and that the mean equivalent diameter of these grains is about 12.1 nm, with values ranging from 4 to 50 nm. Moreover, the ferroelectric orthorhombic phase is stable in substrate-free HZO membranes, indicating that strain from the substrate is not responsible for maintaining the polar phase. It is also demonstrated that HZO capacitors prepared on flexible substrates are highly uniform, stable, and robust. These freestanding membranes provide a viable platform for the exploration of HZO polymorphic films with complex structures and pave the way to flexible nanoelectronics.

Light: Science & Applications, Published online: 11 April 2022; doi:10.1038/s41377-022-00777-w

Cs₂NaInCl₆ double perovskite was codoped with Sb³⁺ (s-electron) and Er³⁺ (f-electrons). Sb³⁺ provides sub-band gap (s to p) optical excitation, which de-excites via both emitting intense blue light and non-radiatively exciting f-electrons of Er³⁺. Then Er³⁺ emits 1540 nm short-wave infrared (SWIR) radiation. Phosphor converted light emitting diode (pc-LED) show stable performance under ambient conditions.

Abstract

Cs₂NaInCl₆ double perovskite is stable, environmentally benign and easy to prepare. But it has a wide band gap (5.1 eV), and therefore, does not show optical and optoelectronic properties in the visible and short-wave infrared (SWIR) region. Here we introduce such functionalities in Cs₂NaInCl₆ by codoping Sb³⁺ (s-electron doping) and Er³⁺ (f-electron doping) ions. Sb³⁺ doping introduces optically allowed 5s ² 5s ¹5p ¹ electronic absorption at the sub-band gap level, which then emits blue photoluminescence with ≈93 % quantum yield. But f-f electronic absorption of Er³⁺ is parity forbidden. Codoping Sb³⁺–Er³⁺, leads to transfer of excitation energy from Sb³⁺ to Er³⁺, yielding SWIR emission at 1540 nm. Temperature (6 to 300 K) dependent photoluminescence measurements elucidate the excitation and emission mechanism. A phosphor converted light emitting diode (pc-LED) fabricated by using the codoped sample emits stable blue and SWIR radiation over prolonged (84 hours) operation at 5.1 V.

Direct growth of wafer-level, ultra-flat graphene without any wrinkles and metallic impurities is implemented on quartz via inhibition of textured SiO _x seed, identification of the critical temperature regime, and in-situ flattening of the substrate surface.

Abstract

The elimination of wrinkles has become a research hotspot in the realm of the chemical vapor deposition growth of graphene, and there have been reliable routes developed for the scenario of catalytic synthesis on metals. Nonetheless, the transfer-free growth of graphene over insulating substrates affording ultra-flatness remains a puzzle. Here, the authors report the direct preparation of ultra-flat graphene on an economical quartz glass substrate at a wafer level, without any wrinkles and metallic impurities, is reported. Density functional theory calculations are employed to establish that graphene adlayer is prone to generate in the presence of textured particulates. In parallel, a critical temperature regime (1443–1453 K), is identified within which graphene adlayer-restrained forming and substrate in-situ flattening could simultaneously occur. The thus-obtained graphene enables the atomically smooth growth of a GaN film with (001) single-crystallinity over the amorphous substrate. This technique is particularly attractive in the context of cost-effective integration of emerging III-nitrides toward exciting applications.

Quantum conductance effects in memristive devices are reviewed, from fundamentals of electrochemical phenomena underlying memristive functionalities to ballistic electronic conduction transport in atomic-sized conductive filaments. Related challenges in nanoscale metrology for the characterization of memristive phenomena at the nanoscale are analyzed together with opportunities and envisioned applications for memristive devices working in the quantum regime.

Abstract

Quantum effects in novel functional materials and new device concepts represent a potential breakthrough for the development of new information processing technologies based on quantum phenomena. Among the emerging technologies, memristive elements that exhibit resistive switching, which relies on the electrochemical formation/rupture of conductive nanofilaments, exhibit quantum conductance effects at room temperature. Despite the underlying resistive switching mechanism having been exploited for the realization of next-generation memories and neuromorphic computing architectures, the potentialities of quantum effects in memristive devices are still rather unexplored. Here, a comprehensive review on memristive quantum devices, where quantum conductance effects can be observed by coupling ionics with electronics, is presented. Fundamental electrochemical and physicochemical phenomena underlying device functionalities are introduced, together with fundamentals of electronic ballistic conduction transport in nanofilaments. Quantum conductance effects including quantum mode splitting, stability, and random telegraph noise are analyzed, reporting experimental techniques and challenges of nanoscale metrology for the characterization of memristive phenomena. Finally, potential applications and future perspectives are envisioned, discussing how memristive devices with controllable atomic-sized conductive filaments can represent not only suitable platforms for the investigation of quantum phenomena but also promising building blocks for the realization of integrated quantum systems working in air at room temperature.

During atomic layer deposition (ALD) ZnO process, H₂O₂ provides an oxygen-rich environment inducing less oxygen vacancies into the ZnO thin film. After applying bias stress of 10 V for 3600 s, the threshold voltage shift for H₂O₂-ZnO thin film transistor is only 0.13 V.

Abstract

ZnO thin films are deposited by atomic layer deposition (ALD) using diethylzinc as the Zn source and H₂O and H₂O₂ as oxygen sources. The oxidant- and temperature-dependent electrical properties and growth characteristics are systematically investigated. Materials analysis results suggest that H₂O₂ provides an oxygen-rich environment so that the oxygen vacancies (V _O) is suppressed, implying a lower carrier concentration and a higher resistivity. The lower growth rate makes it possible for the ZnO thin films to grow along the lower surface energy direction of <002>, leading to a lower Hall mobility. Furthermore, the ZnO semiconductor is integrated into thin film transistor (TFT) devices and the electrical properties are analyzed. The TFT with H₂O₂-ZnO grown at 150 °C shows good electrical properties, such as a high field-effect mobility of 10.7 cm² V^–1 s^–1, a high ratio I _on/I _off of 2 × 10⁷, a sharp subthreshold swing of 0.25 V dec^–1, and a low trapping state (N _trap) of 2.77 × 10¹² eV^–1 cm^–2, which provides a new pathway to optimize the performance of metal-oxide electronics.