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abstract: photovoltaic Effect? Haiyang Zou, Chunli Zhang, Hao Xue, Zhiyi Wu, and Zhong Lin Wang* +School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332 0245, United States +Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, P.R. China The photovoltaic effect in p n junctions has been widely studied and used to work as renewable energy sources for portable devices and various appliances. Due to the band gap energy of semiconductors in p n junctions, carriers are excited by light and then are separated by an internal electric field built in the interface of the junctions, so that the light can be efficiently absorbed and converted into electricity. Nevertheless, the open circuit voltage is limited by the band gap energy of the semiconductors. The bulk photovoltaic (BPV) effect, caused by the asymmetric distribution of photoexcited carriers in momentum space, can generate a large photovoltage under uniform illumination, which is far beyond its band gap of the corresponding semiconductor. However, the power generation efficiency is very low due to the unusually low short circuit current, and the materials that exhibit the BPV effect are generally wide band gap noncentrosymmetric materials. Yang et al. reported the flexo photovoltaic (FPV) effect in which the BPV effect can be induced by strain gradients in any semiconductor through the flexoelectric effect. The authors claimed that the strain gradients create very large photovoltaic currents from centrosymmetric single crystals. A silicon made atomic force microscopy (AFM) tip with a typical radius size of only 8 nm was pressed on a single crystal with a size of more than 0.5 cmx 0.5 cmwith a thickness of more than 0.5 mm to induce strain gradients on the bulk materials. They claimed "the current increases by more than a factor of 100 when the loading force increased from 1 to 18 mN" (actually, from Figure S12A, the current increased by only 3.39 times when the force increased from 1 to 15 mN) The nanosized AFM tip was treated as an ideal rigid spherical indenter, and the Hertzian model was applied to calculate the strain and strain gradient in order to confirm this AFM tip could generate the flexoelectric effect in bulk materials. The author claimed that the flexo photovoltaic effect was discovered as the photocurrent has a dependence on crystallographic orientation and the light polarization with an amplitude of less than 2 pA. Above all, they believed this FPV effect played a dominating role in the increased photocurrent under strains. Here, we focus on four aspects to discuss the data, mechanisms, and conclusions presented in Yang's paper and clarify some facts. First, the experimental design is unreasonable as many other issues involved are not properly addressed when the authors drew the conclusions. Second, the physical models adopted for the calculation analysis are irrational, and the results are based on unrealistic assumptions that far exceed the properties of any materials known. Third, the proposed physical model is based on speculation, and the experimental results do not fully support the physical model. Fourth, the statements made in the discussions and conclusions sections are misleading. Therefore, there are many questions and doubts to be clarified, and many statements need to be corrected. We provide some advice on both experimental methods and theoretical models toward the exact quantification of the FPV effect, which are fundamental and critical, to avoid misleading the scientific community and the public readers by the work from Yang and co authors. More details are elaborated as follows. The Unreasonable Experimental Measurement and Design. Flexoelectricity effect is a coupling effect between electric polarization and strain gradient in centrosymmetric material, which was discovered several decades ago. Studies of flexoelectricity in solids have been scarce due to the extremely small magnitude of this effect in bulk samples. The current studies are mainly focused on materials/structures at the nanoscale as the large strain gradients in nanomaterials can lead to a strong flexoelectric effect. Strain gradients are inversely proportional to size, so thin films are the obvious place to look for flexoelectric effects. Because whenever the thickness exceeds some critical value, the materials may relieve the stress by a formation of misfit dislocations or by twinning, whether the materials are single crystals or not. Yang et al. applied a point force exerted by the tip of a custom made photoelectric atomic force microscope on the bulk materials with a size of more than 0.5 cmx 0.5 cm with a thickness of more than 0.5 mm to induce strain gradients on the bulkmaterials. Yang et al. did not try to do any experiment to verify whether the flexoelectric effect has been induced or not or measure the strength of the flexoelectric effect induced by the strains. Such important data are missing, and we cannot conclude that the AFM tip can generate the flexoelectric effect on these materials. Every material and every sample is different, including various Young's modules, surface roughness, defects, and it is inappropriate to guess that the effect can be obtained by comparing with other papers unless it has been verified experimentally. However, the polarization induced by the applied force has never been measured and verified. A conductive AFM probe was employed for both applying pressure and acting as a conductive electrode. Such a design involves many complicated problems and errors. Smooth surfaces, even those polished, are never perfectly flat on a microscopic scale due to asperities. They are rough, with sharp or rugged projections (Figure 1a,b) Single crystals are never atomically flat after the processes of manufacturing and cutting.
author_names: Haiyang Zou

Chunli Zhang

Hao Xue

Zhiyi Wu

Zhong Lin Wang
title: Boosting the Solar Cell Efficiency by Flexo photovoltaic Effect?
venue: ACS nano
year: 2019

abstract: Poking a semiconductor Noncentrosymmetric crystal structure can lead to a peculiar kind of charge separation under illumination called the bulk photovoltaic (BPV) effect. Solar cells made of such materials, however, typically have low efficiency. Yang et al. expanded the class of materials capable of exhibiting the BPV effect by making ordinarily centrosymmetric materials, such as SrTiO3 and TiO2, lose their inversion symmetry. The authors accomplished this by applying a point force on the surface of the material. This induced a strain gradient and the loss of inversion symmetry, resulting in large photovoltaic currents under illumination. The mechanism, dubbed the flexo photovoltaic effect, is expected to apply to most semiconductors. Science, this issue p. 904 A bulk photoelectric effect is induced in centrosymmetric semiconductors by applying a point force on the materials' surfaces. It is highly desirable to discover photovoltaic mechanisms that enable enhanced efficiency of solar cells. Here we report that the bulk photovoltaic effect, which is free from the thermodynamic Shockley Queisser limit but usually manifested only in noncentrosymmetric (piezoelectric or ferroelectric) materials, can be realized in any semiconductor, including silicon, by mediation of flexoelectric effect. We used either an atomic force microscope or a micrometer scale indentation system to introduce strain gradients, thus creating very large photovoltaic currents from centrosymmetric single crystals of strontium titanate, titanium dioxide, and silicon. This strain gradient induced bulk photovoltaic effect, which we call the flexo photovoltaic effect, functions in the absence of a p n junction. This finding may extend present solar cell technologies by boosting the solar energy conversion efficiency from a wide pool of established semiconductors.
author_names: Ming-Min Yang

Dong Jik Kim

Marin Alexe
title: Flexo photovoltaic effect
venue: Science
year: 2018

abstract: Lithium ion batteries (LIBs) work under sophisticated external force field and its electrochemical properties could be modulated by strain. Owing to the electro mechanical coupling, the change of micro local structures can greatly affect lithium (Li) diffusion rate in solid state electrolytes and electrode materials of LIBs. In this study, we find that strain gradient in bilayer graphene (BLG) significantly affects Li diffusion barrier, which is termed as the flexo diffusion effect, through first principles calculations. The Li diffusion barrier substantially decreases/increases under the positive/negative strain gradient, leading to the change of Li diffusion coefficient in several orders of magnitude at 300 K. Interestingly, the regulation effect of strain gradient is much more significant than that of uniform strain field, which can have a remarkable effect on the rate performance of batteries, with a considerable increase in the ionic conductivity and a slight change of the original material structure. Moreover, our ab initio molecular dynamics simulations (AIMD) show that the asymmetric distorted lattice structure provides a driving force for Li diffusion, resulting in oriented diffusion along the positive strain gradient direction. These findings could extend present LIBs technologies by introducing the novel strain gradient engineering.
author_names: Gao Xu

Feng Hao

Mouyi Weng

Jia-wang Hong

Feng Pan

Daining Fang
title: Flexo diffusion effect: the strong influence on lithium diffusion induced by strain gradient
year: 2020

abstract: Lithium ion batteries (LIBs) work under a sophisticated external force field and the electrochemical properties could be modulated by strain. Owing to electro mechanical coupling, the change of micro local structures can greatly affect the lithium (Li) diffusion rate in solid state electrolytes and the electrode materials of LIBs. In this study, we found, through first principles calculations, that the strain gradient in bilayer graphene (BLG) significantly affects the Li diffusion barrier, which is termed as the flexo diffusion effect. The Li diffusion barrier substantially decreases/increases under a positive/negative strain gradient, leading to a change of Li diffusion coefficient of several orders of magnitude at 300 K. Interestingly, the regulation effect of strain gradient is much more significant than that of a uniform strain field, which can have a remarkable effect on the rate performance of batteries, with a considerable increase in the ionic conductivity and a slight change of the original material structure. Moreover, our ab initio molecular dynamics simulations (AIMD) show that the asymmetric distorted lattice structure provides a driving force for Li diffusion, resulting in oriented diffusion along the positive strain gradient direction. We predict the new phenomenon of a flexo diffusion effect from a theoretical calculation aspect, these findings could extend present LIB technologies by introducing a novel strain gradient engineering.
author_names: Gao Xu

Feng Hao

Mouyi Weng

Jiawang Hong

Feng Pan

Daining Fang
title: Strong influence of strain gradient on lithium diffusion: flexo diffusion effect.
venue: Nanoscale
year: 2020

author_names: Yunyun Gong

Chen Chen

Faqiang Zhang

Xiang He

Huarong Zeng

Qunbao Yang

Yongxiang Li

Zhiguo Yi
title: Ferroelectric photovoltaic and flexo photovoltaic effects in (1 x (Bi 0.5 Na 0.5 )TiO 3 x BiFeO 3 systems under visible light
year: 2020

author_names: Ronggang Cai

Vlad-Andrei Antohe

Bernard Nysten

Luc Piraux

Alain M Jonas
title: Thermally Induced Flexo Type Effects in Nanopatterned Multiferroic Layers
year: 2020

abstract: In this work, the mechanical aspect of a compact prototype of transhumeral prosthesis with flexo extension and prone supination movements is presented. The torque and maximum speed of the elbow and wrist joints were established based on activities of daily living. Lengths, circumferences, and maximum weight were adjusted to anthropometric measurements of the Latin American population. Materials and transmission mechanisms were selected using property pondering matrices. Safety factors and displacements were checked by means of numerical simulation. The parts were manufactured by machining and 3D printing. The mechanism was assembled and validated using the design loads. The mechanism measures 7 cm from the elbow to the arm coupling. It can move objects up to 450 g with retention and up to 2 kg without retention, with maximum speeds of 30 rpm and 20 rpm for the elbow and wrist joints, respectively. The ranges of movement are flexion, 130deg; extension, 30deg; pronation, 90deg; supination, 90deg. The prototype is compact enough to be used in cases of long amputation. The speeds, ranges of motion and load capabilities are suitable for most activities of daily life.
author_names: Pablo Tapia

Marco Ciaccia

Ivan Iglesias
title: Compact Transhumeral Prosthesis Mechanism with Integrated Flexo Extension and Prone Supination Movements
venue: 2020 IEEE ANDESCON
year: 2020

abstract: In this study we have aimed to present statistical evaluation of changes in the surface roughness of the solid tone areas on CtP flexo printing plate in dependence of print run. Changes in flexo plate surface roughness is influenced by the plate making process, printing pressure and speed, the interaction with the ink and printing substrate and by the wear of the printing plate due to long print runs. Thus, the aim of this research was to evaluate the changes in the amplitude surface roughness parameters (Ra, Rp, Rv and Rz) between the printing plates used for different print runs. In the experiment, we have employed three sets of CtP flexo printing plates, for three colors cyan, magenta and blue. The first set was not used for printing, the second was used to print 7 rolls of substrate and the third is used to print 11 rolls of printing substrate. The amplitude surface roughness parameters were compared by applying independentsamples t test method using the software SPSS (Statistical Package for Social Science) with a 0.05 significance level. Statistical analysis revealed that the difference between the amplitude surface roughness parameters measured on CtP flexo printing plates are significant with a 95% confidence level, whereas the statistical significance pointed out that with the longer print runs, the difference will be strongly expressed.
author_names: Sandra Dedijer

Novi Sad Serbia Design

Magdolna Pal

Ivana Tomic

Stefan Poljak

Zivko Pavlovic

Ivana Juric

Neda Milic Kerestes
title: Statistical analysis of the influence of print run on surface roughness of digital flexo printing plates' solid tone areas
year: 2020

abstract: El acero inoxidable es un material con una respuesta no lineal y con endurecimiento por deformacion, lo que condiciona su diseno con respecto al acero al carbono. El CSM es un nuevo metodo de dimensionamiento que tiene en cuenta el endurecimiento por deformacion en secciones poco esbeltas. SE ha publicado recientemente una propuesta para incluir este efecto en el dimensionamiento a pandeo de pilares comprimidos. En este trabajo se pretende extender dicho metodo a pilares de acero inoxidable flexo comprimidos
author_names: Shoghi Moreno

Nur Esteban
title: Propuesta de diseno de pilares de acero inoxidable sometidos a flexo compresion utilizando el Continuous Strengh Method
year: 2020

abstract: In the treatment of cervical pain, several techniques that are commonly used cannot indicate the patient painintensity, but only can identify structural damage. The evaluation of this pain intensity can be achieved analyzing the cervical joint kinematic variables of the three movements: flexo extension, flexo lateral and rotation. In this work we will study the reliability of the photogrammetry technique through a low cost camera, i.e. Kinect V1. The Kinect camera will acquire kinematic parameters of the flexo extension movement from the neck joint. We will use artificial vision and depth/color image techniques to obtaining the trajectories of the technical (and anatomical) markers. A Kalman filter is employed to correct the continuous tracking of the technical markers to obtaining the spatial coordinates of each marker. The data is obtained from seven test subjects, men and women physically healthy. The ages of the testsubjects are between 17 and 40 years. We compute the kinematic parameters of angular velocity, angular acceleration and angular displacement, associated with the spatial coordinates of each technical marker. Then, we obtain the parameters of reliability and correlation between tests through the mean standard error, the multiple correlation index and the Pearsoncorrelation indexes, commonly used for clinical analysis. The high values of these correlation indexes let us to remark the reliability of our methodology.
author_names: Kleber Vicente

William Andres Garzon Venegas

Christian Vasconez

Ivan Zambrano
title: Analisis de Flexo extension del Cuello Mediante el Uso de Vision Artificial
year: 2020