All-silicon multidimensionally-encoded optical physical unclonable functions for integrated circuit anti-counterfeiting.

Integrated circuit anti-counterfeiting based on optical physical unclonable functions (PUFs) plays a crucial role in guaranteeing secure identification and authentication for Internet of Things (IoT) devices. While considerable efforts have been devoted to exploring optical PUFs, two critical challenges remain: incompatibility with the complementary metal-oxide-semiconductor (CMOS) technology and limited information entropy. Here, we demonstrate all-silicon multidimensionally-encoded optical PUFs fabricated by integrating silicon (Si) metasurface and erbium-doped Si quantum dots (Er-Si QDs) with a CMOS-compatible procedure. Five in-situ optical responses have been manifested within a single pixel, rendering an ultrahigh information entropy of 2.32 bits/pixel. The position-dependent optical responses originate from the position-dependent radiation field and Purcell effect. Our evaluation highlights their potential in IoT security through advanced metrics like bit uniformity, similarity, intra- and inter-Hamming distance, false-acceptance and rejection rates, and encoding capacity. We finally demonstrate the implementation of efficient lightweight mutual authentication protocols for IoT applications by using the all-Si multidimensionally-encoded optical PUFs.

Anti-reflection effect of high refractive index polyurethane with different light trapping structures on solar cells.

The textured surfaces to reduce light reflectivity by using acid-alkali chemical etching and SiNx films are generally necessary for commercial crystalline silicon solar cells. However, this etching process requires a large amount of environmentally harmful acid-alkali solution and has limited options for texture and size. To overcome these disadvantages, a new anti-reflection strategy is proposed in this study, which is using soft nanoimprint lithography to prepare the textured structures on the outside of the SiNx films. The polyurethane with a high refractive index of 1.64 is selected as the texture material, and different templates are selected to prepare it into different light trapping structures, including positive-inverted pyramids, inverted lace cones, and positive-inverted moth-eye nanostructures allowing for easy customization of the textured structures. The finite element simulation and experiments demonstrate that these light trapping structures have a wide spectrum anti-reflection performance in visible and near-infrared bands. With the back surface of the commercial passivated emitter rear contact (PERC) bi-facial solar cells as the imprint substrates, some light trapping structures can reduce the surface weighted average light reflectivity (Rw) at the band of 300-1200 nm from 18.31% to less than 10% and the optimal structures can reduce Rw to 8.71%. This anti-reflection strategy can also be applied to thin-film solar cells and crystalline silicon solar cells of other structures, such as HIT, Topcon, Perovskite/c-Si tandem, and so forth, which shows great development potential.

Silicon-Nanomembrane-Based Broadband Synaptic Phototransistors for Neuromorphic Vision.

Neuromorphic vision has been attracting much attention due to its advantages over conventional machine vision (e.g., lower data redundancy and lower power consumption). Here we develop synaptic phototransistors based on the silicon nanomembrane (Si NM), which are coupled with lead sulfide quantum dots (PbS QDs) and poly(3-hexylthiophene) (P3HT) to form a heterostructure with distinct photogating. Synaptic phototransistors with optical stimulation have outstanding synaptic functionalities ranging from ultraviolet (UV) to near-infrared (NIR). The broadband synaptic functionalities enable an array of synaptic phototransistors to achieve the perception of brightness and color. In addition, an array of synaptic phototransistors is capable of simultaneous sensing, processing, and memory, which well mimics human vision.

Solution epitaxy of polarization-gradient ferroelectric oxide films with colossal photovoltaic current.

Solution growth of single-crystal ferroelectric oxide films has long been pursued for the low-cost development of high-performance electronic and optoelectronic devices. However, the established principles of vapor-phase epitaxy cannot be directly applied to solution epitaxy, as the interactions between the substrates and the grown materials in solution are quite different. Here, we report the successful epitaxy of single-domain ferroelectric oxide films on Nb-doped SrTiO3 single-crystal substrates by solution reaction at a low temperature of ~200 oC. The epitaxy is mainly driven by an electronic polarization screening effect at the interface between the substrates and the as-grown ferroelectric oxide films, which is realized by the electrons from the doped substrates. Atomic-level characterization reveals a nontrivial polarization gradient throughout the films in a long range up to ~500 nm because of a possible structural transition from the monoclinic phase to the tetragonal phase. This polarization gradient generates an extremely high photovoltaic short-circuit current density of ~2.153 mA/cm2 and open-circuit voltage of ~1.15 V under 375 nm light illumination with power intensity of 500 mW/cm2, corresponding to the highest photoresponsivity of ~4.306×10-3 A/W among all known ferroelectrics. Our results establish a general low-temperature solution route to produce single-crystal gradient films of ferroelectric oxides and thus open the avenue for their broad applications in self-powered photo-detectors, photovoltaic and optoelectronic devices.

Optogenetics-Inspired Fluorescent Synaptic Devices with Nonvolatility.

Given the synergy of optogenetics and bioimaging in neuroscience, it is possible for light to simultaneously modulate and visualize synaptic events of optoelectronic synaptic devices, which are building blocks of a neuromorphic computing system with optoelectronic integration. Here we demonstrate the realization of the simultaneous modulation and visualization of synaptic events by using optically stimulated synaptic devices based on the heterostructure of fluorescent silicon quantum dots (Si QDs) and monolayer molybdenum disulfide (MoS2). The charge-transfer-enabled photogating effect of the Si QDs/MoS2 heterostructure leads to the nonvolatility of the synaptic devices, which exhibit important synaptic functionalities and synchronous fluorescence upon optical stimulation. An array of the Si QDs/MoS2 optoelectronic synaptic devices is well-employed to mimic robust neural population coding. Defective devices in this array may be pinpointed by the absence of their fluorescence. This work has an important implication for the development of synaptic devices facilitating the system-level diagnosis and device-level positioning of a neuromorphic computing system.

Interface-Enhanced SiOx/Ru Heterocatalysts for Efficient Electrochemical Water Splitting.

Developing a bifunctional electrocatalyst with remarkable performance viable for overall water splitting is increasingly essential for industrial-scale renewable energy conversion. However, the current electrocatalyst still requires a large cell voltage to drive water splitting due to the unsuitable adsorption/desorption capacity of reaction intermediates, which seriously hinders the practical application of water splitting. Herein, a unique SiOx/Ru nanosheet (NS) material was proposed as a high-performance electrocatalyst for overall water splitting. The SiOx/Ru NSs show superior performance in the hydrogen evolution reaction with a low overpotential of 23 mV (@ 10 mA cm-2) and excellent stability for nearly 200 h (@ 10 mA cm-2) in 1 M KOH. By means of the introduction of SiOx, it is beneficial for balancing the local charge density of the surrounding Ru sites. The suitable electronic coupling between the d-band electrons of Ru and the adsorbed species effectively balances the adsorption and desorption of reaction intermediates on the surface. As a result, the catalyst also exhibits overall water splitting activity with a cell voltage of only 1.496 V to reach the current density of 10 mA cm-2. The present work opens up a new strategy for designing high-performance electrocatalysts for water splitting.

Highly Efficient Energy Transfer from Silicon to Erbium in Erbium-Hyperdoped Silicon Quantum Dots.

Erbium-doped silicon (Er-doped Si) materials hold great potential for advancing Si photonic devices. For Er-doped Si, the efficiency of energy transfer (ηET) from Si to Er3+ is crucial. In order to achieve high ηET, we used nonthermal plasma to synthesize Si quantum dots (QDs) hyperdoped with Er at the concentration of ~1% (i.e., ~5 × 1020 cm-3). The QD surface was subsequently modified by hydrosilylation using 1-dodecene. The Er-hyperdoped Si QDs emitted near-infrared (NIR) light at wavelengths of ~830 and ~1540 nm. An ultrahigh ηET (~93%) was obtained owing to the effective energy transfer from Si QDs to Er3+, which led to the weakening of the NIR emission at ~830 nm and the enhancement of the NIR emission at ~1540 nm. The coupling constant (γ) between Si QDs and Er3+ was comparable to or greater than 1.8 × 10-12 cm3·s-1. The temperature-dependent photoluminescence and excitation rate of Er-hyperdoped Si QDs indicate that strong coupling between Si QDs and Er3+ allows Er3+ to be efficiently excited.

Molecular beam epitaxy growth and scanning tunneling microscopy study of 2D layered materials on epitaxial graphene/silicon carbide.

Since the advent of atomically flat graphene, two-dimensional (2D) layered materials have gained extensive interest due to their unique properties. The 2D layered materials prepared on epitaxial graphene/silicon carbide (EG/SiC) surface by molecular beam epitaxy (MBE) have high quality, which can be directly applied without further transfer to other substrates. Scanning tunneling microscopy and spectroscopy (STM/STS) with high spatial resolution and high-energy resolution are often used to study the morphologies and electronic structures of 2D layered materials. In this review, recent progress in the preparation of various 2D layered materials that are either monoelemental or transition metal dichalcogenides on EG/SiC surface by MBE and their STM/STS investigations are introduced.

Solution-Processed Synaptic Memristors Based on Halide Perovskite Nanocrystals.

Exploring new materials and structures to construct synaptic devices represents a promising route to fundamentally approach novel forms of computing. Nanocrystals (NCs) of halide perovskites possess unique charge transport characteristics, i.e., ionic-electronic coupling, holding considerable promise for energy-efficient and reconfigurable artificial synapses. Herein, we report solution-processed thin-film memristors from all-inorganic CsPbBr3 perovskite NCs, functioning as an electrically programmable analog memory with good stability. The devices are demonstrated to successfully emulate a number of essential synaptic functions with low power consumption, including reversible potentiation and depression, short-term plasticity (STP), paired-pulse facilitation (PPF), and long-term plasticity (LTP), such as spike-number-dependent plasticity (SNDP), spike-rate-dependent plasticity (SRDP), spike-timing-dependent plasticity (STDP), and spike-voltage-dependent plasticity (SVDP). It is proposed that a coupled capacitive and inductive phenomenon originating from charge trapping and ion migration in CsPbBr3 NC films, controlled by the amplitude and timing of the programming pulses, defines the degree of synaptic plasticity. A transition emerges from the fast trap-related capacitive regime to a slow ionic inductive regime, which enables continuous change of the film resistance and the magnitude of the electronic current, analogous to the synaptic weight modulation in biological synapses.

Recent progress and future prospects on halide perovskite nanocrystals for optoelectronics and beyond.

As an emerging new class of semiconductor nanomaterials, halide perovskite (ABX3, X = Cl, Br, or I) nanocrystals (NCs) are attracting increasing attention owing to their great potential in optoelectronics and beyond. This field has experienced rapid breakthroughs over the past few years. In this comprehensive review, halide perovskite NCs that are either freestanding or embedded in a matrix (e.g., perovskites, metal-organic frameworks, glass) will be discussed. We will summarize recent progress on the synthesis and post-synthesis methods of halide perovskite NCs. Characterizations of halide perovskite NCs by using a variety of techniques will be present. Tremendous efforts to tailor the optical and electronic properties of halide perovskite NCs in terms of manipulating their size, surface, and component will be highlighted. Physical insights gained on the unique optical and charge-carrier transport properties will be provided. Importantly, the growing potential of halide perovskite NCs for advancing optoelectronic applications and beyond including light-emitting devices (LEDs), solar cells, scintillators and X-ray imaging, lasers, thin-film transistors (TFTs), artificial synapses, and light communication will be extensively discussed, along with prospecting their development in the future.

Optoelectronic Artificial Synaptic Device Based on Amorphous InAlZnO Films for Learning Simulations.

Neuromorphic computing, which mimics brain function, can address the shortcomings of the "von Neumann" system and is one of the critical components of next-generation computing. The use of light to stimulate artificial synapses has the advantages of low power consumption, low latency, and high stability. We demonstrate amorphous InAlZnO-based light-stimulated artificial synaptic devices with a thin-film transistor structure. The devices exhibit fundamental synaptic properties, including excitatory postsynaptic current, paired-pulse facilitation (PPF), and short-term plasticity to long-term plasticity conversion under light stimulation. The PPF index stimulated by 375 nm light is 155.9% when the time interval is 0.1 s. The energy consumption of each synaptic event is 2.3 pJ, much lower than that of ordinary MOS devices and other optical-controlled synaptic devices. The relaxation time constant reaches 277 s after only 10 light spikes, which shows the great synaptic plasticity of the device. In addition, we simulated the learning-forgetting-relearning-forgetting behavior and learning efficiency of human beings under different moods by changing the gate voltage. This work is expected to promote the development of high-performance optoelectronic synaptic devices for neuromorphic computing.

Neuromorphic device based on silicon nanosheets.

Silicon is vital for its high abundance, vast production, and perfect compatibility with the well-established CMOS processing industry. Recently, artificially stacked layered 2D structures have gained tremendous attention via fine-tuning properties for electronic devices. This article presents neuromorphic devices based on silicon nanosheets that are chemically exfoliated and surface-modified, enabling self-assembly into hierarchical stacking structures. The device functionality can be switched between a unipolar memristor and a feasibly reset-able synaptic device. The memory function of the device is based on the charge storage in the partially oxidized SiNS stacks followed by the discharge activated by the electric field at the Au-Si Schottky interface, as verified in both experimental and theoretical means. This work further inspired elegant neuromorphic computation models for digit recognition and noise filtration. Ultimately, it brings silicon - the most established semiconductor - back to the forefront for next-generation computations.

Recent progress of heterostructures based on two dimensional materials and wide bandgap semiconductors.

Recent progress in the synthesis and assembly of two-dimensional (2D) materials has laid the foundation for various applications of atomically thin layer films. These 2D materials possess rich and diverse properties such as layer-dependent band gaps, interesting spin degrees of freedom, and variable crystal structures. They exhibit broad application prospects in micro-nano devices. In the meantime, the wide bandgap semiconductors (WBS) with an elevated breakdown voltage, high mobility, and high thermal conductivity have shown important applications in high-frequency microwave devices, high-temperature and high-power electronic devices. Beyond the study on single 2D materials or WBS materials, the multi-functional 2D/WBS heterostructures can promote the carrier transport at the interface, potentially providing novel physical phenomena and applications, and improving the performance of electronic and optoelectronic devices. In this review, we overview the advantages of the heterostructures of 2D materials and WBS materials, and introduce the construction methods of 2D/WBS heterostructures. Then, we present the diversity and recent progress in the applications of 2D/WBS heterostructures, including photodetectors, photocatalysis, sensors, and energy related devices. Finally, we put forward the current challenges of 2D/WBS heterostructures and propose the promising research directions in the future.

Recent Progress on the Scanning Tunneling Microscopy and Spectroscopy Study of Semiconductor Heterojunctions.

The band alignment, interface states, interface coupling, and carrier transport of semiconductor heterojunctions (SHs) need to be well understood for the design and fabrication of various important semiconductor structures and devices. Scanning tunneling microscopy (STM) with high spatial resolution and scanning tunneling spectroscopy (STS) with high energy resolution are significantly contributing to the understanding on the important properties of SHs. In this work, the recent progress on the use of STM and STS to study lateral, vertical and bulk SHs is reviewed. The spatial structures of SHs with atomically flat surface have been examined with STM. The electronic band structures (e. g., the band offset, interface state, and space charge region) of SHs are measured with STS. Combined with the spatial structures and the tunneling spectra features, the mechanism for the carrier transport in the SH may be proposed.

CKAP4 Antibody-Conjugated Si Quantum Dot Micelles for Targeted Imaging of Lung Cancer.

At present, various fluorescent nanomaterials have been designed and synthesized as optical contrast agents for surgical navigation. However, there have been no reports on the preparation of fluorescent contrast agents for lung cancer surgery navigation using silicon quantum dots (Si QDs). This study improved and modified the water-dispersible Si QD micelles reported by Pi et al. to prepare Si QD micelles-CKAP4. The data showed that the Si QD micelles-CKAP4 were spherical particles with a mean hydrodiameter of approximately 78.8 nm. UV-visible absorption of the Si QD micelles-CKAP4 ranged from 200 to 500 nm. With an excitation wavelength of 330 nm, strong fluorescence at 640 nm was observed in the fluorescence emission spectra. Laser confocal microscopy and fluorescence microscopy assay showed that the Si QD micelles-CKAP4 exhibited good targeting ability to lung cancer cells and lung cancer tissues in vitro. The in vivo fluorescence-imaging assay showed that the Si QD micelles-CKAP4 was metabolized by the liver and excreted by the kidney. In addition, Si QD micelles-CKAP4 specifically targeted lung cancer tissue in vivo compared with healthy lung tissue. Cytotoxicity and hematoxylin and eosin staining assays showed that the Si QD micelles-CKAP4 exhibited high biosafety in vitro and in vivo. Si QD micelles-CKAP4 is a specifically targeted imaging agent for lung cancer and is expected to be a fluorescent contrast agent for lung cancer surgical navigation in the future.

Low-temperature processed tantalum/niobium co-doped TiO2electron transport layer for high-performance planar perovskite solar cells.

A low-temperature preparation process is significantly important for scalable and flexible devices. However, the serious interface defects between the normally used titanium dioxide (TiO2) electron transport layer (ETL) obtained via a low-temperature method and perovskite suppress the further improvement of perovskite solar cells (PSCs). Here, we develop a facile low-temperature chemical bath method to prepare a TiO2ETL with tantalum (Ta) and niobium (Nb) co-doping. Systematic investigations indicate that Ta/Nb co-doping could increase the conduction band level of TiO2and could decrease the trap-state density, boosting electron injection efficiency and reducing the charge recombination between the perovskite/ETL interface. A superior power conversion efficiency of 19.44% can be achieved by a planar PSC with a Ta/Nb co-doped TiO2ETL, which is much higher than that of pristine TiO2(17.60%). Our achievements in this work provide new insights on low-temperature fabrication of low-cost and highly efficient PSCs.

Enhanced photoluminescence of silicon quantum dots in the presence of both energy transfer enhancement and emission enhancement mechanisms assisted by the double plasmon modes of gold nanorods.

We propose a scheme utilizing the double plasmon modes of gold nanorods (GNRs) to efficiently increase the Förster resonant energy transfer (FRET) efficiency and enhance the photoluminescence (PL) of Si quantum dots (Si QDs) nearby. Detailed PL and decay dynamics studies are performed for the hybrid nanostructures composed of metallic nanoparticles (MNPs) coated with a Si QD-absorbed silica shell. Plasmon enhanced FRET between Si QDs has been observed and proposed as the third enhancement mechanism for the plasmon-enhanced photoluminescence in addition to excitation enhancement and emission enhancement mechanisms. A maximum FRET efficiency of 46.3% is obtained, which is enhanced by a factor of 8.7 compared to that of samples without MNPs. The dependence of the energy transfer efficiency and the enhancement of the acceptor emission on the surface plasmon resonance (SPR) wavelength, metal-QD distance and QD ratio is examined. The FRET enhancement mechanism dominates when the coupling of plasmon-donor is much stronger than that of plasmon-acceptor with a high acceptor/donor ratio.

Correction: Enhanced photoluminescence of silicon quantum dots in the presence of both energy transfer enhancement and emission enhancement mechanisms assisted by the double plasmon modes of gold nanorods.

[This corrects the article DOI: 10.1039/D1NA00287B.].

Perovskite-Enhanced Silicon-Nanocrystal Optoelectronic Synaptic Devices for the Simulation of Biased and Correlated Random-Walk Learning.

Silicon- (Si-) based optoelectronic synaptic devices mimicking biological synaptic functionalities may be critical to the development of large-scale integrated optoelectronic artificial neural networks. As a type of important Si materials, Si nanocrystals (NCs) have been successfully employed to fabricate optoelectronic synaptic devices. In this work, organometal halide perovskite with excellent optical asborption is employed to improve the performance of optically stimulated Si-NC-based optoelectronic synaptic devices. The improvement is evidenced by the increased optical sensitivity and decreased electrical energy consumption of the devices. It is found that the current simulation of biological synaptic plasticity is essentially enabled by photogating, which is based on the heterojuction between Si NCs and organometal halide perovskite. By using the synaptic plasticity, we have simulated the well-known biased and correlated random-walk (BCRW) learning.