Fig. 1. Schematic illustration of local characterization of in-plane piezoelectricity and vertical piezoelectricity. In-plane piezoelectricity (piezo) (d11, d22) of ultrathin materials is the planar electromechanical couple behavior, where the applied stress and produced piezoelectric potential are located at the in-plane of exposed lattice plane. Vertical piezoelectricity (d33) focus on electromechanical interaction occurred in the vertical axis, which is perpendicular to the surface of materials. The high-precision deformation actuator can be implemented using accurate positioning of the materials surface by vertical inverse piezoelectricity. Credit: X. Wang, X. He, H. Zhu, L. Sun, W. Fu, X. Wang, L. C. Hoong, H. Wang, Q. Zeng, W. Zhao, J. Wei, Z. Jin, Z. Shen, J. Liu, T. Zhang, Z. Liu, Subatomic deformation driven by vertical piezoelectricity from CdS ultrathin films. Sci. Adv. 2, e1600209 (2016). Copyright © 2016 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC). DOI:10.1126/sciadv.1600209. 

Piezoelectricity (aka the piezoelectric effect) occurs within certain materials – crystals (notably quartz), some ceramics, bone, DNA, and a number of proteins – when the application of mechanical stress or vibration generates electric charge or alternating current (AC) voltage, respectively. (Conversely, piezoelectric materials can vibrate when AC voltage is applied to them.) The piezoelectric effect has a significant range of uses, including sound production and detection, generation of high voltages and electronic frequencies, atomic resolution imaging technologies (e.g., scanning tunneling and atomic force microscopy), and actuators for highly accurate positioning of nanoscale objects – the last being crucial for fundamental research and industrial applications. That being said, subatomic scale positioning still presents a number of challenge. Recently, however, researchers at Nanyang Technological University, Singapore, Chinese Academy of Sciences, Suzhou, and Duke University, Durham demonstrated vertical piezoelectricity at the atomic scale (three to five space lattices) using ultrathin cadmium sulfide (CdS) films. The researchers determined a vertical piezoelectric coefficient (d₃₃) three times that of bulk CdS using in situ scanning Kelvin force microscopy and single and dual ac resonance tracking piezoelectric force microscopy, leading them to conclude that their findings have a number of critical roles in the design of next-generation sensors and microelectromechanical devices.

Prof. Zheng Liu discussed the paper that he, Dr. Ting Zhang and their colleagues published in Science Advances, describing a series of challenges they faced starting with using chemical vapor deposition to synthesize 2~3 nm cadmium sulfide (CdS) thin films. "The vertical piezoelectricity, or d₃₃, is the key parameter in piezoelectric materials for the fabrication of actuators used to position objects with extreme accuracy – down to the atomic scale in a broad range of cutting-edge equipment such as atomic force microscopy and scanning tunneling microscopy," Liu tells Phys.org. "Moreover, high-performance ultrathin piezoelectric materials are crucial for the constructing ultra-high resolution and flexible electromechanically coupled devices."

Prior to this study, Liu points out, only a few studies reported the synthesis of atomic thin piezoelectric materials by a wet chemical method, examples of which include CdS and cadmium selenide (CdSe) nanoplatelets. "It's a significant challenge to produce high-quality and atom-thin piezoelectric materials," he adds. "In this research, the main challenge in synthesizing ultrathin piezoelectric CdS films via chemical vapor deposition" (or CVD) "lays in the selection of precursors and how to optimize the reaction parameters, such as growing temperature and time."

The scientists were then faced with demonstrating d₃₃ vertical piezoelectricity at the atomic scale using ultrathin cadmium sulfide thin films. "When the thickness of materials reaches the nanoscale level," Liu explains, "it's very difficult to verify the piezoelectric effect and determine its values because of the coupling effect from the substrate – and surface geometries may affect the measurements at atomic limits as well." For example, he illustrates, sample surface roughness reaches tens of picometers, which is the same scale with the vertical electromechanical response for materials. 

Fig. 2. Spectroscopic characterization of CdS thin film. (A) Energy (E) band structure in the vicinity of the Γ-point of the Brillouin zone, showing the photon emission process. (B and C) PL spectrum of CdS thin film from points i and ii marked in (C) with plus signs, showing strong band edge emission (506 nm) of CdS ultrathin film and defect-related emission (595 nm). (C) Optical image of CdS thin film with a rounded microparticle at its center. a.u., arbitrary units. (D and E) PL mapping at an emission of 514 nm with a different scale bar, demonstrating high uniformity and homogeneity of CdS thin films at the outside area. (F) PL mapping at the 595-nm emission, indicating that the defect-related emission only occurs at the thicker CdS microparticle. Credit: X. Wang, X. He, H. Zhu, L. Sun, W. Fu, X. Wang, L. C. Hoong, H. Wang, Q. Zeng, W. Zhao, J. Wei, Z. Jin, Z. Shen, J. Liu, T. Zhang, Z. Liu, Subatomic deformation driven by vertical piezoelectricity from CdS ultrathin films. Sci. Adv. 2, e1600209 (2016). Copyright © 2016 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC). DOI:10.1126/sciadv.1600209. 

Lastly – and reminiscent of the challenge in demonstrating d₃₃ vertical piezoelectricity at the atomic scale using CdS thin films – the researchers had to determine the CdS film vertical piezoelectric coefficient with in situ scanning Kelvin force microscopy (SKFM) and single and dual AC resonance tracking piezoelectric force microscopy (DART-PFM). "The quality of ultrathin piezoelectric CdS is the key to obtaining a reliable vertical piezoelectric coefficient." Liu notes. "Some characterization tools like Raman and photoluminescence spectroscopies can help us to identify the CdS sample and confirm its high quality. Also, because of the geometrical vibrations of the CdS samples, the atomic force microscopy characterization should be carefully carried out in order to make sure our conclusions are solid." This required the researchers to examine many SKFM and DART-PFM samples using to reach a solid conclusion about vertical piezoelectric behavior in CdS ultrathin films.

Liu comments that addressing these challenges required innovative techniques. "For the first time, we successfully synthesized high-quality atomic thin CdS films using CVD, and we demonstrated vertical piezoelectricity of these films at the atomic scale of 3~5 space lattices" (a space, or crystal, lattice being a periodically repeating two- or three-dimensional array of points or particles) "and observed the vertical piezoelectric domains. More importantly," Liu continues, "our work shows an enhanced vertical piezoelectricity in CdS ultrathin films at a level three times larger than the CdS bulk counterpart, as well as higher than most of traditional piezoelectric materials." These results imply non-trivial piezoelectric behavior at atomic limits for a certain class of materials – which has not yet been well explored – and inspires the search for two-dimensional free-standing layered piezoelectric materials that are only one atom thick.

Liu points out that their findings shed light on the design of next-generation sensors, actuators and microelectromechanical devices, in that piezoelectric materials are the most important component for such devices. Specifically, he says that their findings provide the opportunity for next-generation sensors and microelectromechanical devices in three ways:

• Flexibility: Ultrathin piezoelectric material materials are naturally like two-dimensional materials in being flexible, allowing them to be conformably used for more complicated electromechanical devices.
• Miniaturization: Ultrathin piezoelectric material materials are a perfect candidate for the fabrication of reduced size, highly integrated devices, especially for mobile phone and wearable devices.
• Inspiration: The study's results will inspire the development of other ultrathin piezoelectric materials, especially two-dimensional piezoelectric materials.