Small Band Gap Research Articles

Diamond/cubic boron nitride (111) heterojunction interface: Rational regulation of high two-dimonsional electron gas

Wide-bandgap diamond exhibits excellent physical and chemical properties, making it an ideal substrate material for developing high-temperature and high-power semiconductor devices and broadening its application prospects in the field of microelectronics and optoelectronic devices. However, because of the Fermi pinning effect, it is difficult to achieve n-type diamond with excellent transport properties. Cubic boron nitride (cBN) has similar structural properties and a smaller lattice mismatch with diamond. Thus, making the two-dimensional electron gas of the diamond/cBN heterojunction provides a new idea to solve the problem of diamond n-incorporation difficulty and small bandgap regulation range. The diamond/cBN heterojunction is applied to study the law of influence factors on the heterojunction band structure, band alignment, and polarization strength. Further, a regulation mechanism for two-dimensional electron gas density is established at heterojunction interfaces, which can help solve the core difficulties of diamond n-type doping, small bandgap regulation range, and high impedance, providing a theoretical basis for realizing diamond power devices with high power density and low power consumption.

Controlling the Phase Distribution of Single Bromide Quasi-2-Dimensional Perovskite Crystals via Solvent Engineering for Pure-Blue Light-Emitting Diodes.

To achieve pure-blue emission (460-470 nm), we manipulate the crystallization process of the quasi-2D perovskite, (PBA)2Csn-1PbnBr3n+1, prepared by a solution process. The strategy involves controlling the distribution of "n" phases with different bandgaps, solely utilizing changes in the precursor's supersaturation to ensure that the desired emission aligns with the smallest bandgap. Adjustments in photoluminescence (PL) wavelength are made by changing the solute concentration and solvent polarity, as these factors heavily influence the diffusion of cations, a crucial determinant for the value of "n". Subsequently, we enhance the PL quantum yield from 31 to 51% at 461 nm using trioctylphosphine oxide (TOPO) as an additive of antisolvent, which passivates halide vacancy and promotes orderly crystal growth, leading to faster carrier transfer between phases. With these strategies, we successfully demonstrate pure-blue LEDs with a turn-on voltage of 3.3 V and an external quantum efficiency of 5.5% at an emission peak of 470 nm with a full-width at half-maximum of 31 nm.

Synergistic charge-transfer dynamics of rigid fused and unfused backbone with donors lead to promising photovoltaic properties of diazaborinine-based chromophores

Herein, a series of diazaborinin based derivatives (FABD1–FABD8) having D-π-D configuration was designed by inserting different types of rigid fused and unfused π-bridges into reference molecule (FABR) to explore their photovoltaic characteristics. To determine the optoelectronic and charge transport characteristics of these analogues, DFT/TD-DFT based calculations were conducted at the MPW1PW91/6-31G (d,p) level. For this, various analyses were performed such as transition density matrix (TDM), global reactivity parameters (GRP), open-circuit voltage (Voc), optical properties (UV–Vis), binding energy (Eb), hole-electron and frontier molecular orbital (FMO) energies. Their insights indicate that a rigid fused π-bridge greatly lowers the band gap as compared to an unfused π-spacers. Particularly, FABD7 showed the lowest energy gap of 2.20 eV owing to the presence of a rigid fused central core as compared to FABR with a band gap of 2.65 eV. Similarly, the entitled designed derivatives demonstrated a redshift absorption in the range of 585.7–704.6 nm while FABR is reported with 558.7 nm in the solvent phase. Furthermore, these derivatives displayed significant Voc with blending of PC61BM acceptor. This observation was also supported by DOS and TDM based insights. Interestingly, FABD7 seemed as a viable photovoltaic material due to its strong optical properties and small band gaps. These results uncover that the designed chromophores may be considered as efficient to enhance the performance of OSCs.

Energy band engineering of graphitic carbon nitride for photocatalytic hydrogen peroxide production

AbstractHydrogen peroxide (H2O2) is one of the 100 most important chemicals in the world with high energy density and environmental friendliness. Compared with anthraquinone oxidation, direct synthesis of H2O2 with hydrogen (H2) and oxygen (O2), and electrochemical methods, photocatalysis has the characteristics of low energy consumption, easy operation and less pollution, and broad application prospects in H2O2 generation. Various photocatalysts, such as titanium dioxide (TiO2), graphitic carbon nitride (g‐C3N4), metal‐organic materials, and nonmetallic materials, have been studied for H2O2 production. Among them, g‐C3N4 materials, which are simple to synthesize and functionalize, have attracted wide attention. The electronic band structure of g‐C3N4 shows a bandgap of 2.77 eV, a valence band maximum of 1.44 V, and a conduction band minimum of −1.33 V, which theoretically meets the requirements for hydrogen peroxide production. In comparison to semiconductor materials like TiO2 (3.2 eV), this material has a smaller bandgap, which results in a more efficient response to visible light. However, the photocatalytic activity of g‐C3N4 and the yield of H2O2 were severely inhibited by the electron‐hole pair with high recombination rate, low utilization rate of visible light, and poor selectivity of products. Although previous reviews also presented various strategies to improve photocatalytic H2O2 production, they did not systematically elaborate the inherent relationship between the control strategies and their energy band structure. From this point of view, this article focuses on energy band engineering and reviews the latest research progress of g‐C3N4 photocatalytic H2O2 production. On this basis, a strategy to improve the H2O2 production by g‐C3N4 photocatalysis is proposed through morphology control, crystallinity and defect, and doping, combined with other materials and other strategies. Finally, the challenges and prospects of industrialization of g‐C3N4 photocatalytic H2O2 production are discussed and envisioned.

Defect Engineering of MoTe2 via Thiol Treatment for Type III van der Waals Heterojunction Phototransistor.

Molybdenum ditelluride (MoTe2) nanosheets have displayed intriguing physicochemical properties and opto-electric characteristics as a result of their tunable and small band gap (Eg ∼ 1 eV), facilitating concurrent electron and hole transport. Despite the numerous efforts devoted to the development of p-type MoTe2 field-effect transistors (FETs), the presence of tellurium (Te) point vacancies has caused serious reliability issues. Here, we overcome this major limitation by treating the MoTe2 surface with thiolated molecules to heal Te vacancies. Comprehensive materials and electrical characterizations provided unambiguous evidence for the efficient chemisorption of butanethiol. Our thiol-treated MoTe2 FET exhibited a 10-fold increase in hole current and a positive threshold voltage shift of 25 V, indicative of efficient hole carrier doping. We demonstrated that our powerful molecular engineering strategy can be extended to the controlled formation of van der Waals heterostructures by developing an n-SnS2/thiol-MoTe2 junction FET (thiol-JFET). Notably, the thiol-JFET exhibited a significant negative photoresponse with a responsivity of 50 A W-1 and a fast response time of 80 ms based on band-to-band tunneling. More interestingly, the thiol-JFET displayed a gate tunable trimodal photodetection comprising two photoactive modes (positive and negative photoresponse) and one photoinactive mode. These findings underscore the potential of molecular engineering approaches in enhancing the performance and functionality of MoTe2-based nanodevices as key components in advanced 2D-based optoelectronics.

Spin–Orbit Coupling Free Nonlinear Spin Hall Effect in a Triangle-Unit Collinear Antiferromagnet with Magnetic Toroidal Dipole

We investigate emergent conductive phenomena triggered by collinear antiferromagnetic orderings. We show that an up-down-zero spin configuration in a triangle cluster leads to linear and nonlinear spin conductivities even without the relativistic spin–orbit coupling; the linear spin conductivity is Drude-type, while the nonlinear spin conductivity has Hall-type characterization. We demonstrate the emergence of both spin conductivities in a breathing kagome system consisting of a triangle cluster. The nonlinear spin conductivity becomes larger than the linear one when the Fermi level lies near the region where a small partial band gap opens. Our results indicate that collinear antiferromagnets with triangular geometry give rise to rich spin conductive phenomena.

Band engineering for building cascading charge transport channels in ternary CdS-based nanorod array photoanode toward efficient photoelectrochemical H2 generation

A novel CdS/CdSe/Cu2O nanorod array (NRA) photoelectrode with cascading charge transport channels was designed for the first time to remarkedly boot photoelectrochemical (PEC) hydrogen generation under visible light illumination with zero bias. This ternary photoelectrode was constructed by modifying CdS nanorod arrays with n-type CdSe and p-type Cu2O nanoparticles, successively. In this novel photoanode, CdSe and Cu2O with small band gap not merely highly heighten the visible light absorption capacity of CdS, but more importantly integrate with CdS to build cascading charge transport channels at the CdS/CdSe and CdSe/Cu2O interfaces, dramatically facilitating the separation and transport efficiency for photoexcited electron and hole pairs. The CdS/CdSe/Cu2O NRA photoelectrode gains an obvious hydrogen generation rate of 193.2 μmol·h−1, 8.2 times higher than that of bare CdS NRA photoelectrode. The remarkably enhanced PEC hydrogen production for the CdS/CdSe/Cu2O NRA photoanode can be ascribed to effective solar energy utilization and charge carrier separation.

Tuning the structural, optical, and dielectric properties of europium-doped barium titanate ceramics

This study explores the optical and dielectric properties of pure and europium-doped barium titanate ceramics with formula Ba1−3x/2EuxTiO3 (x=0,0.005,0.015and0.025\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$x=0,0.005, 0.015\ ext{ and }0.025$$\\end{document}). The materials were synthesized using the solid-state reaction method. The influence of Eu + 3 ion incorporation on the crystal structure was examined using XRD, which indicated the formation of a cubic phase. Lattice parameter measurements showed excellent agreement with various theoretical models for predicting the lattice constant, with an average error of only 0.20%. UV–visible spectroscopy analysis is utilized to investigate optical characteristics, revealing a decrease in the optical band gap from 2.65 to 2.77 eV post-Eu doping, which is notably lower than the bulk barium titanate value of 3.2 eV. Materials with smaller band gaps are more suited for optoelectronic applications like photodetectors and light-emitting diodes (LEDs). The dielectric response of europium-doped barium titanate was studied across a frequency range of 1 kHz to 2 MHz and a temperature range of 77 to 300 K, showing remarkable stability. The frequency-dependent dielectric analysis revealed that the dielectric constant initially increased with increasing doping concentration, but decreased at higher concentrations. These results suggest that europium-doped barium titanate holds significant promise for future electronic device applications due to its enhanced optical and dielectric properties.

Impact on preferred orientation and crystal strain behavior of nanocrystal anatase-TiO2 by X-ray diffraction technique

The primary goal of this research outline is the conversion of titanium isopropoxide (TTIP) at 50.0 °C to form a high crystalline preferred oriented predominant (101) with a low crystal strain of 90.0 % anatase-TiO2 phase. With the interval of time, the peptizing agent (IP) reacts to an acidic aqueous medium and preferential growth of anatase-TiO2 has been identified. Exhaustive recombination in X-ray diffraction (XRD) analysis ensures the crystal strain, lattice volume, lattice parameters, d-spacing and crystallite size. UV–vis-NIR showed maximum absorption of 320.0 nm with 0.78 a.u. at blue shift for decreasing nano-size and smaller bandgap 3.0313 eV of larger dimensions anatase-TiO2. The preferred orientation also revealed by nanobeam diffraction (NBD) of TEM enables qualitative lattice type and highly crystal orientated predominant (101) plane 5.60 nm−1 in a diffraction pattern imposed on 200.0 kv parallel electron bean from LaB6 filament.

Amplifying Photochromic Response in Tungsten Oxide Films with Titanium Oxide and Polyvinylpyrrolidone.

Tungsten oxide (WO3) is known for its photochromic properties, making it useful for smart windows, displays, and sensors. However, its small bandgap leads to rapid recombination of electron-hole pairs, resulting in poor photochromic performance. This study aims to enhance the photochromic properties of WO3 by synthesizing hexagonal tungsten oxide via hydrothermal synthesis, which increases surface area and internal hydrates. Titanium oxide (TiO2) was adsorbed onto the tungsten oxide to inject additional charges and reduce electron-hole recombination. Additionally, polyvinylpyrrolidone (PVP) was used to improve dispersion in organic solvents, allowing for the fabrication of high-quality films using the doctor blade method. Characterization confirmed the enhanced surface area, crystal structure, and dispersion stability. Reflectance and transmittance measurements demonstrated significant improvements in photochromic properties due to the composite structure. These findings suggest that the introduction of TiO2 and PVP to tungsten oxide effectively enhances its photochromic performance, broadening its applicability in various advanced photochromic applications.

Synthesis of sub‐micron sized SiC particles with high defect density by using polytetrafluoroethylene as an additive

AbstractA new preparation process for silicon carbide (SiC) powder is developed. In the Si–(C) –PTFE–Ar system, the grain size and morphology of the product 3C–SiC were controlled by adding a carbon source (graphite) and changing the percentage of polytetrafluoroethylene (PTFE) (0%, 10%, and 20%). The experimental results showed that the SiC powders prepared using a molar ratio of 1:1 silicon powder to graphite, plus 20% PTFE have a uniform particle size distribution (∼130 nm), a lamellar structure made of spherical particle stacking, a small bandgap (1.80 eV), a high carrier concentration, and a large number of lattice defects. These properties are expected to increase the electrical conductivity of 3C–SiC and decrease its thermal conductivity, thus providing a promising feedstock preparation option for SiC thermoelectric materials. In addition, the mechanism of PTFE in the preparation of SiC reactions was studied in detail.

Uncovering fabrication approach impact on photocatalytic ciprofloxacin (CIP) antibiotic degradation of brookite TiO2

Solar-driven antibiotic degradation by semiconductor photocatalysis technique offers a promising route to tackle with the environmental issue we are currently facing. Herein, three kinds of brookite TiO2, TiO2-Glycerol (GL)/NaOH (OH−), TiO2-Ethylene glycol (EG)/Ethylenediamine (EDA), and TiO2- Ethylene glycol (EG)/NaOH (OH−), are prepared by changing fabrication approaches from as-prepared two different Ti-polyol precursors. The composition and structure of the above brookite nanocrystals is studied in detail, and the effects of different synthesis methods on their optical properties, surface areas and wettability, and photocatalytic degradation activities are discussed. The photocatalytic ciprofloxacin (CIP) remediation evaluation shows that the TiO2-EG/EDA enriched amine group confirmed by FTIR and XPS measurements has the highest dark adsorption as well as subsequent photocatalytic degradation activities compare with the other two types of brookite. Subsequently, the unveiling on how the TiO2-EG/EDA effectively degrades the CIP molecules is achieved. Indeed, on the basis of a series of characterizations, the good CIP adsorption capacity resulted from the weakest hydrophilicity, the largest specific surface area, the unique chemisorption effect by surface amine group, combining with the smallest band gap, the fastest photogenerated charge transport speed, the longest photogenerated electron lifetime and the most active sites, contribute to the rapid degradation of CIP upon the TiO2-EG/EDA. In addition, the CIP degradation pathway is proposed by HRMS analysis, and the EPI suite program predicts the intermediate molecules have no biotoxicity for the environment. It is expected this work could provide useful reference for highly-efficient photocatalytic antibiotics-degradation in terms of experimental design and materials fabrication.

Rice hull-derived biochar with photocatalytic activity for efficient degradation of oxytetracycline in water under sunlight

Photocatalytic degradation is an effective strategy for the removal of environmental pollutants. However, the development of low-cost and high-efficiency photocatalysts continues to present a significant challenge. This work reported the fabrication of a composite photocatalyst directly from waste biomass (rice hulls) in a single step, which efficiently degraded oxytetracycline (OTCs) under sunlight. The morphology, microstructures and photoelectric features of the fabricated materials were subjected to comprehensive investigation using characterization techniques. The results found that the improved photocatalytic activity in sunlight was attributed to the complexation of rice hull-derived biochar and nano-ZnO generated in the preparation, which exhibited favourable adsorption, conductivity characteristics and small bandgap. The photocatalytic efficiency was observed to reach 72.6 % in 120 min under the irradiation of a 100 W xenon lamp. The effect of natural waters, degradation kinetics and degradation TOC analysis were also examined. The dominant agent responsible for the degradation of OTCs was the •O2− radical. Furthermore, the principal photocatalytic degradation pathway of OTCs was identified with the assistance of UHPLC/MS.

Precursor-Confined Chemical Vapor Deposition of 2D Single-Crystalline SexTe1-x Nanosheets for p-Type Transistors and Inverters.

Two-dimensional (2D) tellurium (Te) is emerging as a promising p-type candidate for constructing complementary metal-oxide-semiconductor (CMOS) architectures. However, its small bandgap leads to a high leakage current and a low on/off current ratio. Although alloying Te with selenium (Se) can tune its bandgap, thermally evaporated SexTe1-x thin films often suffer from grain boundaries and high-density defects. Herein, we introduce a precursor-confined chemical vapor deposition (CVD) method for synthesizing single-crystalline SexTe1-x alloy nanosheets. These nanosheets, with tunable compositions, are ideal for high-performance field-effect transistors (FETs) and 2D inverters. The preformation of Se-Te frameworks in our developed CVD method plays a critical role in the growth of SexTe1-x nanosheets with high crystallinity. Optimizing the Se composition resulted in a Se0.30Te0.70 nanosheet-based p-type FET with a large on/off current ratio of 4 × 105 and a room-temperature hole mobility of 120 cm2·V-1·s-1, being eight times higher than thermally evaporated SexTe1-x with similar composition and thickness. Moreover, we successfully fabricated an inverter based on p-type Se0.30Te0.70 and n-type MoS2 nanosheets, demonstrating a typical voltage transfer curve with a gain of 30 at an operation voltage of Vdd = 3 V.

Low‐Power Charge Trap Flash Memory with MoS2 Channel for High‐Density In‐Memory Computing

AbstractWith the rise of on‐device artificial intelligence (AI) technology, the demand for in‐memory comptuing has surged for data‐intensive tasks on edge devices. However, on‐device AI requires high‐density, low‐power memory‐based computing to efficiently handle large data volumes. Here, this study proposes a reliable multilevel, high gate‐coupling ratio memory device with MoS2 channel tailored for high‐density 3D NAND Flash‐based in‐memory computing. The MoS2 channel, featured by its small bandgap and high‐mobility, facilitates reliable memory window of approximately 8 V thanks to erase operation through hole injection. This not only suppresses vertical charge loss but also alleviates the burden on voltage generator circuits, indicating the suitability of MoS2 as channel material for 3D NAND Flash architecture. Additionally, a low‐k (≈2.2) tunneling layer deposited via initiated chemical vapor deposition increases the gate‐coupling ratio, thereby reducing the operating voltage. Utilizing Au nanoparticles as the charge storage layer, MoS2 memory devices show synaptic plasticity with 6‐bit, endurance (104 cycles), read disturbance (105 cycles), and retention times (105 s). Furthermore, device‐to‐system simulations for neural networks based on MoS2‐memory devices have successfully achieved a fingerprint recognition of 95.8%. These results provide the foundation to develop multi‐bit MoS2‐memory devices for AI accelerators and 3D NAND Flash memory.

Microstructural and optical properties of nanostructured Al/Ag co-doped ZnO thin films (AgxAl0.03ZnO0.97-x) by spray pyrolysis for optoelectronic applications

Al/Ag co-doped ZnO (AgxAl0.03ZnO0.97-x, x = 0, 0.02, 0.04, 0.06, 0.08) thin films were prepared using a facile spray pyrolysis deposition route, and their microstructural and optical properties were investigated. Dislocation density reduced and film crystallinity improved with increasing Ag content. This correlated with strain relaxation (∼10−4) and crystallite size enlargement (>23 nm) at high Ag concentrations, as determined by the Scherrer, Williamson-Hall, Halder-Wagner, and Wagner-Agua approximations. The small Urbach energy (<321 meV) and narrow band gap (∼3 eV) observed at low Ag concentration (4 at.%) indicate less structural defects and disorder, and facilitate exciton dissociation with minimum recombination, respectively. The Moss, Herve and Vandamme, and the Ravindra and Gupta models exhibited high correlation between refractive index values (2.09–2.35). Thus, this study demonstrates the enormous potential of Al/Ag co-doping to advance the properties of ZnO films to suit the fabrication of optoelectronic devices, and create opportunities for commercialization.

Two-Dimensional GeC/MXY (M = Zr, Hf; X, Y = S, Se) Heterojunctions Used as Highly Efficient Overall Water-Splitting Photocatalysts.

Hydrogen generation by photocatalytic water-splitting holds great promise for addressing the serious global energy and environmental crises, and has recently received significant attention from researchers. In this work, a method of assembling GeC/MXY (M = Zr, Hf; X, Y = S, Se) heterojunctions (HJs) by combining GeC and MXY monolayers (MLs) to construct direct Z-scheme photocatalytic systems is proposed. Based on first-principles calculations, we found that all the GeC/MXY HJs are stable van der Waals (vdW) HJs with indirect bandgaps. These HJs possess small bandgaps and exhibit strong light-absorption ability across a wide range. Furthermore, the built-in electric field (BIEF) around the heterointerface can accelerate photoinduced carrier separation. More interestingly, the suitable band edges of GeC/MXY HJs ensure sufficient kinetic potential to spontaneously accomplish water redox reactions under light irradiation. Overall, the strong light-harvesting ability, wide light-absorption range, small bandgaps, large heterointerfacial BIEFs, suitable band alignments, and carrier migration paths render GeC/MXY HJs highly efficient photocatalysts for overall water decomposition.

Efficient and economically affordable TiO2 nanotube-based ternary photocatalysts for CO2 conversion boosted by NiO nanoparticles and carbon quantum dots

Photocatalytic CO2 conversion, in general, requires the incorporation of expensive and noble materials to achieve a highly efficient conversion rate. Herein, the TNT/NiO/CQD ternary nanocomposites without any expensive materials and material modification are fabricated for the demonstration of efficient and economically affordable photocatalytic CO2 conversion, to our best knowledge, for the first time. The TNT-based ternary photocatalysts are successfully prepared by a simple anodization and immersion process. The TNT/NiO/CQD ternary nanocomposites produce 1.47 μmol·cm-2·h-1 (≈c.a. 2834 μmol·g-1·h-1) of CH4 as the sole product under AM 1.5 illumination. This is five times higher performance than that of bare TNT and the performance is comparable to that of any other TiO2-based unitary, binary, or ternary photocatalysts. This highly enhanced performance results from effective junction formation between TNT and NiO NP, and the dual role of CQDs as a light absorber and charge transporter/collector in the system. The formation of Z-scheme at the TNT/NiO interface leads to efficient charge transfer via a TiO2/NiO bond. The light absorption of the photocatalysts is enhanced and extended by decorated CQDs due to their small band gap. The charge transfer and collection are further improved by charge depletion at the TNT/CQD interface and the charge transfer at the NiO/CQD interface. This work provides a possible model for the realization of efficient and economically affordable photocatalytic CO2 conversion.

Wide absorption spectrum and rapid response time of PEC photodetectors based on MoS2–Se nanocomposites

Photoelectrochemical (PEC) photodetectors convert light into electrical signals via PEC processes, providing high sensitivity, positioning them as promising candidates for next-gen optoelectronic devices. Over the past few years, there has been a significant interest in the field of photodetection regarding 2D materials, owing to their advantageous properties. In this study, the PEC photodetection of hydrothermally synthesized MoS2–Se nanocomposites have been systematically tested. Structural analysis confirms the coexistence of MoS2 and Se phases in the synthesized nanocomposites with crystallite sizes in the nanoscale dimension. Optical studies indicate that MoS2–Se nanocomposites exhibit broadened absorption spectrum extending to UV region, unlike pure MoS2 which absorbs solely in the visible region. This enhances their sensitivity for wide-band photodetectors with a small bandgap (1.01–1.34 eV). Optoelectronic study revealed superior performance of MoS2–Se thin film PEC device with efficient photoresponse with a responsivity of 38.3 μA/W and fast response time of 0.12 s. The novelty of this research resides in the integration of these unique features in MoS2–Se nanocomposites for PEC photodetectors, explored for the first time. The innovative aspect of this study involves employing spin coating to deposit 2D MoS2–Se thin films, which enhances control over film thickness and uniformity, thereby resulting in improved responsivity and photocurrent.

Noncollinear Electric Dipoles in a Polar Chiral Phase of CsSnBr3 Perovskite.

Polar and chiral crystal symmetries confer a variety of potentially useful functionalities upon solids by coupling otherwise noninteracting mechanical, electronic, optical, and magnetic degrees of freedom. We describe two phases of the 3D perovskite, CsSnBr3, which emerge below 85 K due to the formation of Sn(II) lone pairs and their interaction with extant octahedral tilts. Phase II (77 K < T < 85 K, space group P21/m) exhibits ferroaxial order driven by a noncollinear pattern of lone pair-driven distortions within the plane normal to the unique octahedral tilt axis, preserving the inversion symmetry observed at higher temperatures. Phase I (T < 77 K, space group P21) additionally exhibits ferroelectric order due to distortions along the unique tilt axis, breaking both inversion and mirror symmetries. This polar and chiral phase exhibits second harmonic generation from the bulk and pronounced electrostriction and negative thermal expansion along the polar axis (Q22 ≈ 1.1 m4 C-2; αb = -7.8 × 10-5 K-1) through the onset of polarization. The structures of phases I and II were predicted by recursively following harmonic phonon instabilities to generate a tree of candidate structures and subsequently corroborated by synchrotron X-ray powder diffraction and polarized Raman and 81Br nuclear quadrupole resonance spectroscopies. Preliminary attempts to suppress unintentional hole doping to allow for ferroelectric switching are described. Together, the polar symmetry, small band gap, large spin-orbit splitting of Sn 5p orbitals, and predicted strain sensitivity of the symmetry-breaking distortions suggest bulk samples and epitaxial films of CsSnBr3 or its neighboring solid solutions as candidates for bulk Rashba effects.