When uniform strain is applied to noncentrosymmetric semiconductor crystals, which are piezoelectric, static polarization charges are induced at the surface. If the applied strain is not uniform, these charges can even be created inside the crystal. The applied strain affects electronic transport and also photonic processes, and thus can be used to tune the material properties statically or dynamically. As a result, two new fields have emerged, namely piezotronics and piezo-phototronics. This article reviews the history of the two fields and gives a perspective on their applications. The articles in this issue of MRS Bulletin highlight progress in these two fields, and this article places this progress into perspective.

Piezotronic and piezo-phototronic devices exhibit high performance and have potential applications especially in next-generation self-powered, flexible electronics and wearable systems. In these devices, a strain-induced piezoelectric field at a junction, contact, or interface can significantly modulate the carrier generation, recombination, and transport properties. This mechanism has been studied based on the theory of piezotronics and piezo-phototronics. Simulation-driven materials design and device improvements have been greatly propelled by the finite element method, density functional theory, and molecular dynamics for achieving high-performance devices. Dynamical piezoelectric fields can also control new quantum states in quantum materials, such as in topological insulators, which pave a new path for enhancing performance and for investigating the fundamental physics of quantum piezotronics and piezo-phototronics.

Third-generation semiconductors, such as ZnO and GaN, exhibit strong piezoelectric polarization due to the lack of inversion symmetry. The piezotronic effect observed in these semiconductors was proposed for tuning carrier transport in electronic devices by utilizing the induced piezoelectric potential as a virtual gate. This novel concept allows effective interactions between micro-/nanoelectronic devices and external mechanical stimuli. Piezotronics provide a promising approach for designing future electronic devices beyond Moore’s Law with potential for developing smart sensors, environment monitoring systems, human–machine interaction elements, and other transducers. In this article, we review recent progress in piezotronics using one-dimensional materials, heterojunctions, and large-scale arrays. We provide guidance for future piezotronic devices based on these materials.

Piezotronics can not only afford control of electronic transport over potential barriers, but the attendant mechanical stress can also influence various physical properties of piezoelectric semiconductors. Stress significantly affects the optical properties of these materials as well as their response toward the chemical environment and magnetic fields. This article focuses on the utilization of piezotronics with regard to these physical parameters for sensor applications. Stress sensors, optical sensors (especially in the ultraviolet range), and sensors for chemicals in gas and liquid phases or magnetic fields via coupled magnetostrictive layers are discussed. The benefits of piezotronics for sensors are highlighted by discussing respective figures of merit.

Electrochemical catalyst design and optimization primarily relies on understanding and facilitating interfacial charge transfer. Recently, piezotronics have emerged as a promising method for tuning the interfacial energetics. The unique band-engineering capability using piezoelectric or ferroelectric polarization could lead to performance gains for electrochemical catalysis beyond what can be achieved by chemical or structural optimization. This article addresses the fundamentals of surface polarization and corresponding band modulation at solid–liquid interfaces. The most recent advances in piezotronic modulations are discussed from multiple perspectives of catalysis, including photocatalytic, photoelectrochemical, and electrochemical processes, particularly for energy-related applications. The concept of piezocatalysis, a direct conversion of mechanical energy to chemical energy, is introduced with an example of mechanically driven water splitting. While still in the early stages, piezotronics is envisioned to become a powerful tool for revolutionizing electrochemical catalysis.

Optoelectronic nanoscale devices have wide applications in chemical, biological, and medical technologies. Improving the performance efficiency of these devices remains a challenge. Performance is mainly dictated by the structure and characteristics of the semiconductor materials. Once a nanodevice is fabricated, its efficiency is determined. The key to improving efficiency is to control the interfaces in the device. In this article, we describe how the piezo-phototronic effect can be effectively utilized to modulate the band at the interface of a metal/semiconductor contact or a p–n junction to enhance the external efficiency of many optoelectronic nanoscale devices such as photodetectors, solar cells, and light-emitting diodes (LEDs). The piezo-phototronic effect can be highly effective at enhancing the efficiency of energy conversion in today’s green and renewable energy technology without using the sophisticated nanofabrication procedures that have high cost and complexity.

This article discusses recent studies of piezotronics and piezo-phototronics of two-dimensional (2D) materials. Two-dimensional semiconductor materials have demonstrated excellent electronic and optoelectronic properties, and these ultrathin materials are candidates for next-generation devices. Among 2D semiconductors, transition-metal dichalcogenides in particular have large in-place piezoelectricity due to the noncentrosymmetry along the armchair direction. A strong coupling of piezoelectric and semiconducting properties has been reported for Schottky contacts and p–n junctions, even in single-layer materials. Since the carrier concentration of ultrathin 2D materials can be easily modulated by external piezocharges, layered composites of ferroelectric/2D materials also show promising piezotronic and piezo-phototronic properties.

The piezophotonic effect is the coupling between piezoelectric properties and photoexcitation, where strain-induced piezopotential modulates and controls the relevant optical process. Specifically, metal ions as activators are capable of responding to photoexcitation and subsequent emission of light, also called mechanoluminescence, in general, and piezoluminescence specifically for piezoelectrics. These phenomena are helpful for understanding the materials fundamentals and conceiving widespread device applications. In this article, we briefly introduce the physical mechanisms of piezophotonics, including piezoluminescence. Selected host materials and metal-ion activators are described for demonstrating the piezophotonic effect. We provide a unified profile and recent prototypical demonstrations of light emission triggered by mechanical stimuli. The devices based on these materials offer the advantages of remote detection, nondestructive analysis, and repeatability, hence they are promising candidates for applications in stress sensing, structural health diagnosis, three-dimensional handwriting, magnetic-optical sensing, energy harvesting, biomedicine, novel light sources, and displays.

Harnessing the nonvolatility of magnetism and the power of electric control, magnetoelectric devices that control magnetism electrically promise to deliver next-generation electronics systems that can store and compute large amounts of information with minimal power consumption and ultrafast processing speed. We highlight progress in magnetoelectric memory and logic prototypes using the voltage-controlled magnetic anisotropy (VCMA) effect. First, important performance metrics of VCMA-based magnetoelectric random access memory (MeRAM) are benchmarked against embedded complementary metal oxide semiconductor and other emerging embedded nonvolatile memories. We then discuss scaling of MeRAM from the physics and materials perspectives of the VCMA effect, as well as the use of magnetoelectric logic devices and circuits to realize new computing paradigms with VCMA. Finally, challenges to realize the full potential of VCMA-based memory and logic are presented: VCMA coefficient of 1000 fJ/V-m for energy-efficient write with low errors and tunneling magnetoresistance of 1000% for high density and low noise margin readout. New approaches for deterministic switching based on VCMA are needed. We share perspectives to address these challenges using new materials and device operation schemes.