Patent Publication Number: US-2019189840-A1

Title: Method of transferring nanostructures and device having the nanostructures

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to methods for transferring nanostructures and devices having the transferred nanostructures. 
     2. Description of Related Art 
     Devices with physical flexibility and stretchability have attracted a great deal of interest for use in wearable electronic technology and large-area electronics, including displays, energy harvesters, energy storage devices, distributed sensor networks, and Internet of Things applications [1]. Moreover, the flexibility is a key factor for enhancing the performance for piezoelectric devices, such as piezoelectric transistors [2], self-powered nanogenerators [3,4], sensors [5,6], and piezo-phototronic effect enhanced solar cells [7,8] and light-emitting diodes [9] driven by the mechanical energy from the environment. One-dimensional semiconductors, i.e., nanorods (NRs) or nanowires (NWs), are promising for flexible device applications, because these structures represent the most effective route for obtaining a high maximum flexion and maintaining high performance under strain and deformation. [3,10-12] Single-crystal III-nitride nanorods are one of the most important semiconductors due to their tunable and direct band gap, good chemical stability, tunable electrical structure, and great piezoelectrical characteristics for a large number of applications, such as piezoelectric nanogenerators,[13] nanolasers,[14,15] photodetectors,[11] photovoltaic cells,[16] and hydrogen generation.[17-19] High-quality single-crystalline III-nitride nanorods are typically epitaxied at high temperatures on rigid single-crystalline Si (111), sapphire, and SiC substrates, but these substrates cannot be adapted for flexible electronics or some applications. For flexible applications, high-quality of GaN nanorods have been grown on highly crystalline single- or few-layer of transferred graphene on bulk substrates,[20,21] and then the Nanorods and graphene could be transferred using the wet-etching method.[22] However, metal contamination [ 23 ] and the complications of the graphene and nanorods transfer processes may degrade the device performance and limit their industrial applications. 
     REFERENCES 
     
         
         
           
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Oehler, et al., Epitaxy of GaN Nanowires on Graphene, Nano Lett. 16 (2016) 4895-4902. [22] C.-H. Lee, Y.-J. Kim, Y. J. Hong, S.-R. Jeon, S. Bae, B. H. Hong, et al., Flexible Inorganic Nanostructure Light-Emitting Diodes Fabricated on Graphene Films, Adv. Mater. 23 (2011) 4614-4619. [23] G. Lupina, J. Kitzmann, I. Costina, M. Lukosius, C. Wenger, A. Wolff, et al., Residual Metallic Contamination of Transferred Chemical Vapor Deposited Graphene, ACS Nano 9 (2015) 4776. [24] A. Mazid Munshi, H. Weman, Advances in semiconductor nanowire growth on graphene, Phys. Status Solidi RRL 7 (2013) 713-726. [25] M. Li, D. Liu, D. Wei, X. Song, D. Wei, A. Thye Shen Wee, Controllable Synthesis of Graphene by Plasma-Enhanced Chemical Vapor Deposition and Its Related Applications, Adv. Sci. 3 (2016) 1600003. [26] T. Detchprohm, K. Hiramatsu, K. Itoh, I. Aksaki, Relaxation Process of the Thermal Strain in the GaN/α-Al 2 O 3  Heterostructure and Determination of the Intrinsic Lattice Constants of GaN Free from the Strain, Jpn. J. Appl. Phys. 31 (1992) L1454. [27] M. A. Reshchikov, H. Morkoç, Luminescence properties of defects in GaN, J. Appl. Phys. 97 (2005) 061301. [28] H.-Y. Chen, H.-W. Lin, C.-H. Shen, S. Gwo, Structure and photoluminescence properties of epitaxially oriented GaN nanorods grown on Si (111) by plasma-assisted molecular-beam epitaxy, Appl. Phys. Lett. 2006, 89, 243105. [29] Q. Li, G. T. Wang, Spatial Distribution of Defect Luminescence in GaN Nanowires, Nano Lett. 10 (2010) 1554-1558. [30] P. Huang, H. Zong, J.-J. Shi, M. Zhang, X.-H. Jiang, H.-X. Zhong, Y et al., Origin of 3.45 eV Emission Line and Yellow Luminescence Band in GaN Nanowires: Surface Microwire and Defect, ACS Nano 9 (2015) 9276-9283. [31] G. T. Hwang, H. Park, J. H. Lee, S. Oh, K. I. Park, M. Byun, et al., Self-powered cardiac pacemaker enabled by flexible single crystalline PMN-PT piezoelectric energy harvester, Adv. Mater. 26 (2014) 4880. [32] C. H. Wang, W. S. Liao, Z. H. Lin, N. J. Ku, Y. C. Li, Y. C. Chen, et al., Optimization of the Output Efficiency of GaN Nanowire Piezoelectric Nanogenerators by Tuning the Free Carrier Concentration, Adv. Energy Mater. 4 (2014) 1400392. [33] S.-J. Tsai, C.-Y. Lin, C.-L. Wang, J.-W. Chen, C.-H. Chen, C.-L. Wu, Efficient Coupling of Lateral Force in GaN Nanorod, Piezoelectric Nanogenerators by Vertically Integrated Pyramided Si Substrate, Nano Energy 37 (2017) 260-267. [34] L. Lin, C.-H. Lai, Y. Hu, Y. Zhang, X. Wang, C. Xu, et al., High output nanogenerator based on assembly of GaN nanowires, Nanotechnology 22 (2011) 475401. [35] Y. Gao, Z. L. Wang, Electrostatic Potential in a Bent Piezoelectric Nanowire. The Fundamental Theory of Nanogenerator and Nanopiezotronics, Nano Lett. 7 (2007) 2499-2505. [36] C. Y. Chen, J. H. Huang, J. Song, Y. Zhou, L. Lin, P. C. Huang, et al., Anisotropic Outputs of a Nanogenerator from Oblique-Aligned ZnO Nanowire Arrays, ACS Nano 5 (2012) 6707-6713. [37] Y. Hu, L. Lin, Y. Zhang, Z. L. Wang, Replacing a Battery by a Nanogenerator with 20 V Output, Adv. Mater. 24 (2011) 110-114. [38] G. Zhu, A. C. Wang, Y. Liu, Y. Zhou, Z. L. Wang, Functional Electrical Stimulation by Nanogenerator with 58 V Output Voltage, Nano Letters, 2012, 12 (6), 3086-309Nano Lett. 12 (2012) 3086-3090. 
           
         
       
    
     SUMMARY OF THE INVENTION 
     In one general aspect, the present invention relates to a method for transferring nanostructures and a device having the nanostructures. 
     According to an embodiment of this invention, a method for transferring nanostructures comprises the steps of: forming a two-dimensional material (2D material) on a first substrate; forming a plurality of nanostructures on the 2D material; bonding a top surface of one or more of nanostructures with a head or a second substrate, and/or shaking the one or more nanostructures with or without a fluid; and separating the one or more nanostructures from the 2D material. 
     According to another embodiment of this invention, a device is provided with nanostructures that are formed by the above-mentioned method. 
     According to another embodiment of this invention, a device is provided with an array comprising one or more layers of light-emitting diodes, piezoelectric transistors, sensors (e.g. piezoelectric pressure sensors, image sensors, biosensors, or piezo-phototronic effect enhanced sensors), nanogenerators, solar cells, piezo-phototronic effect enhanced solar cells, or chemical reaction cells (e.g. photoelectrochemical water-splitting cells, piezoelectric effect enhanced photoelectrochemical water-splitting cells, or fuel cells). In this text, the term “light-emitting diode (LED)” refers to a semiconductor light source for lighting, displaying, optical sensing, and/or other applications and such devices may include: LEDs, laser diodes (LDs), micro or pixel LEDs, micro or pixel LDs, piezo-phototronic effect enhanced LEDs or LDs, or piezo-phototronic effect enhanced micro or pixel LEDs or LDs. Each of them comprises one or more nanostructures that are formed on a three-dimensional (3D) orientated morphology of a 2D material on a first substrate and then separated from the 2D material and transferred to a second substrate or a fluid or a container, wherein the orientations of the nanostructures disposed on the second substrate are random or are controlled to have one or more oblique angles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flow chart showing a method for transferring nanostructures in accordance with an embodiment of this invention. 
         FIG. 2A  is a schematic cross-sectional diagram showing a 2D material grown on a first substrate and vertically aligned nanostructures grown on the 2D material. 
         FIG. 2B  is a schematic cross-sectional diagram showing a 2D grown on a first substrate and obliquely aligned nanostructures grown on the 2D material. 
         FIG. 3  is a schematic cross-sectional diagram showing p-n junction nanostructures grown on 2D material on a first substrate in accordance with an embodiment of this invention. 
         FIG. 4  is a schematic cross-sectional diagram showing p-n junction nanostructures grown on 2D material on a first substrate in accordance with another embodiment of this invention. 
         FIG. 5  is a schematic cross-sectional diagram showing p-n junction nanostructures with DBRs grown on 2D material on a first substrate in accordance with another embodiment of this invention. 
         FIG. 6A  is a cross-sectional view illustrating an LED array in accordance with an embodiment of this invention. 
         FIG. 6B  is a cross-sectional view illustrating an LED array in accordance with an embodiment of this invention. 
         FIG. 6C  is a cross-sectional view illustrating an LED array in accordance with an embodiment of this invention. 
         FIG. 6D  is a cross-sectional view illustrating an LED array in accordance with an embodiment of this invention. 
         FIG. 6E  is a cross-sectional view illustrating a light distribution of the LED array of  FIGS. 6C and 6D . 
         FIG. 6F  is a cross-sectional view illustrating a light distribution of the LED array of  FIGS. 6C and 6D . 
         FIG. 6G  is a cross-sectional view illustrating a light distribution of the LED array of  FIGS. 6C and 6D . 
         FIG. 7A  is a schematic diagram showing a method for transferring obliquely aligned GaN nanorods in accordance with an embodiment of this invention. 
         FIG. 7B  is a picture showing the obliquely aligned GaN nanorods being easily separated from graphene in accordance with an embodiment of this invention. 
         FIGS. 8A-8J  show SEM images of surface morphology of flat or three-dimensional (3D) oriented graphene nanosheets grown on a Si (100) substrate and obliquely aligned GaN nanorods ( 8 F) and random orientated InGaN/GaN nanorods ( 8 I) grown on the 3D oriented graphene nanosheets and upper/outer shell graphene further grown on the GaN nanorods ( 8 J) and the 3D oriented graphene nanosheets after the GaN nanorods are separated in accordance with embodiments of this invention. 
         FIG. 9  shows TEM images of the obliquely aligned GaN nanorods grown on the 3D oriented nanosheets in accordance with embodiments of this invention. 
         FIG. 10  shows PL characterizations of produced single-crystalline GaN nanorods in accordance with an embodiment of this invention. 
         FIG. 11A  shows pictures of a flexible nanogenerator integrated with the transferred GaN nanorods in their original, bending, and straightening states for power generation in accordance with an embodiment of this invention. 
         FIG. 11B  is a chart illustrating the correlation between a magnified output voltage and the bending conditions of the nanogenerator shown in  FIG. 11A . 
         FIG. 11C  is a chart illustrating an output voltage and current of the nanogenerator shown in  FIG. 11A . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The detailed description of the present invention will be discussed in the following embodiments, which are not intended to limit the scope of the present invention, but can be adapted for other applications. While drawings are illustrated in details, it is appreciated that the quantity of the disclosed components may be greater or less than that disclosed, except expressly restricting the amount of the components. Wherever possible, the same or similar reference numbers are used in drawings and the description to refer to the same or like parts. It should be noted that any drawing presented are in simplified form and are not to precise scale. In reference to the disclosure herein, for purposes of convenience and clarity only, directional terms are used with respect to the accompanying drawing and should not be construed to limit the scope of the invention in any manner. 
     This invention discloses methods for transferring nanostructures and devices having the transferred nanostructures. 
       FIG. 1  is a flow chart showing a method for transferring nanostructures in accordance with an embodiment of this invention. In this text, the term “nanostructure” refers to a structure of intermediate size between microscopic and molecular structures, such as nanorods, nanowires, nanocones, nanotubes, nanodisks, nanoshells, nanoparticles, and the likes or combinations of one or more shapes of nanostructures. 
     Referring to  FIG. 1 , the method comprises the steps of: step  101 , forming a 2D material on a first substrate; step  102 , forming a plurality of nanostructures on the 2D material; step  103 , bonding a top surface of one or more of the plurality of nanostructures with a head or a second substrate; and step  104 , separating the one or more nanostructures from the 2D material. 
     The first substrate can be any kind of substrate such as semiconductor (e.g., silicon, SiGe, or SiC), metal (e.g., Titanium), insulator (e.g., sapphire, glass, or quartz), and combinations thereof. The 2D material is a crystalline or low-crystalline material consisting of a single or few or multi layers of atoms. The layered 2D materials feature strong in-plane covalent bonding and weak intra-plane bonding. Preferably, the 2D material comprises graphene, 2D allotropes (e.g. graphene, phosphorene, germanene, silicone, borophene), transition metal dichalcogenide (e.g., WSe 2 , WS 2 , or MoS 2 ), 2D group-IV materials, 2D group-V materials, 2D oxides, or combinations thereof. The graphene family comprises graphene, hexagonal boron nitride (hBN, white graphene), fluorographene, and graphene oxide. 
     Method for forming the 2D material on the first substrate and method for forming the nanostructures may include, but is not limited to, physical vapor deposition (PVD), plasma-assisted molecular beam epitaxial (PA-MBE), chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), atomic layer deposition (ALD), Metal-organic Chemical Vapor Deposition (MOCVD), wet chemical method, or hydrothermal method. 
     The inventor has discovered that the grain size of the 2D material affects its morphology, and high-quality nanostructures can be achieved by using low-crystalline 2D material as a growing substrate. In addition, if the 2D material is low-crystalline, its surface morphology can be flat or 3D oriented depending on the time, surface roughness, or surface morphology of the first substrate for growing the 2D material. 
       FIG. 2A  is a schematic cross-sectional diagram in accordance with an embodiment of this invention. Referring to  FIG. 2A , a low-crystalline 2D material  12  having a flat surface morphology  121  is grown on a first substrate  10  with a short growing period and then vertically aligned nanostructures  14  are grown on flat surface morphology  121  of the 2D material  12 . 
       FIG. 2B  is a schematic cross-sectional diagram in accordance with another embodiment of this invention. Referring to  FIG. 2B , a low-crystalline 2D material  12  having a 3D oriented surface morphology  122  is grown on a first substrate  10  with a long growing period and then obliquely aligned nanostructures  14  are grown on 3D oriented surface morphology  122  of the 2D material  12 . Typically the low-crystalline 2D material  12  with 3D oriented surface morphology  122  consists of few or multi layers of atoms. The low-crystalline 2D material  12  refers to a 2D material with an average small grain size, e,g., less than 500 nm. In an embodiment, the surface of the first substrate  10  is patterned or textured before or after the formation of the 2D material  12  to control the orientations and/or positions of the nanostructures. 
     Referring to  FIGS. 2A and 2B , the grain size of the 2D material  12  and/or process parameters for growing the nanostructures  14  are controlled so that each nanostructure  14  has a bottom coupled to the 2D material  12  and the diameter of the bottom is smaller than the diameter of middle of nanostructure  14 . This morphology is quite helpful for separating the nanostructures  14  from the 2D material  12 . The diameter of the nanostructure  14  can be controlled by the growth substrate (e.g. the grain size or grain shape or crystallization of the 2D material, and/or surface pattern or texture morphology of the first substrate), and/or the process parameters for growing the nanostructures  14 . In an embodiment, the ratio of the diameter of middle to the diameter of bottom of nanostructure  14  ranges between 2:1 and 1000:1. In an embodiment, the grain size of the 2D material  12  ranges between 3 nm and 500 nm. In an embodiment, the average grain size of the 2D material  12  is less than 500 nm. In an embodiment, the average grain size of the 2D material  12  is less than 200 nm. In an embodiment, the average grain size of the 2D material  12  is less than 100 nm. In an embodiment, the surface of the first substrate  10  is patterned or textured to control the diameter of the nanostructure  14 . 
     Referring to  FIG. 2B , it is observed that the 3D oriented surface morphology  122  are grown from the grain boundaries of the 2D material  12  with a small grain size, e,g., less than 500 nm. In one embodiment, the orientations of nanostructures  14  grown on the 3D oriented surface morphology  122  of the 2D material  12  are random with no specific direction. Referring to  FIG. 2B , at least a portion of nanostructures  14  are obliquely formed on 3D oriented surface morphology  122  of the 2D material  12  by a glancing-angle epitaxy and the orientations of the portion of nanostructures  14  are controlled by processing parameters of the glancing-angle epitaxy. In an embodiment, the surface morphology of the 2D material  12  is controlled to be partially flat and partially 3D oriented, so that a portion of the nanostructures  14  formed on the 2D material  12  are vertically aligned and the other of that are obliquely aligned. 
     In an embodiment, the top surface of the plurality of nanostructures  14  is bonded with a head (not shown) by electrostatic force. The head is used for selectively picking up and transferring one or more nanostructures  14  from the first substrate  10  to the second substrate. The head may comprise a protruded electrode and a dielectric layer covering the protruded electrode. The head has a monopolar or bipolar electrode configuration. In addition, an electrode is formed on the top surfaces of nanostructures  14  before the transfer. The transferring procedure includes that the head contacts or approaches the electrode of one or more nanostructures to be transferred and a voltage is applied to the protruded electrode to create an electrostatic force on the one or more nanostructures. The one or more nanostructures  14  are then picked up to separate from the 2D material  12 . The nanostructures  14  can be picked up and transferred individually, in groups, or as the entire array. This invention can achieve large-scale (e.g., more than or equal to centimeter-scale) in a single transfer process of nanostructures  14 . In particular, this invention can achieve extra large-scale (e.g., more than or equal to meter-scale) in a single transfer process of nanostructures  14  by placing multi-wafers of nanostructures  14  side by side, and then separate the whole nanostructures  14  from the multi-wafers at a time. The one or more nanostructures  14  are then released onto a second substrate or transferred into a fluid (e.g. gas, water or viscous solution) or a container with the fluid for production of a device (e.g. chemical reaction cells). 
     In an embodiment, a bonding layer is utilized during the formation and/or transfer of the nanostructures  14  to the second substrate. The bonding layer may be made of metals, solders, thermoplastic polymers, or combinations thereof. If necessary, the bonding layer can be electrically conductive. In an embodiment, the top surfaces of one or more nanostructures  14  are bonded to the bonding layer of head or second substrate under heat and/or pressure, and then the one or more nanostructures  14  are lifted to separate from the 2D material. The bonding layer may be a permanent layer or an intermediate layer and may undergo a phase change (e.g., solid to liquid or liquid to solid) during the formation of device or transfer of nanostructures  14 . The bonding layer may be patterned before the transfer. In an embodiment, the one or more nanostructures  14  are then released into a fluid (e.g. gas, liquid, or viscous solution) for production of a device (e.g. a chemical reaction cell). 
     In an embodiment, an adhesive layer is utilized during the formation and/or transfer of the nanostructure to the second substrate. A head or a second substrate includes an adhesive layer, which is made of suitable materials such as adhesive polymers. In an embodiment, the top surfaces of one or more nanostructures  14  are bonded to the adhesive layer of head or second substrate under heat and/or pressure, and then the one or more nanostructures  14  are lifted to separate from the 2D material. The adhesive layer may be a permanent layer or an intermediate layer removed after the transfer. The adhesive layer may be patterned before the transfer. In an embodiment, the one or more nanostructures  14  are then released into a fluid (e.g. gas, water, or viscous solution). While embodiments of the present invention are described with specific regard to separating the nanostructures by bonding, adhering or electrostatic attraction, it is to be appreciated that embodiments of the invention are not so limited and that certain embodiments may also be applicable to other separating means. Alternatively, a shaking procedure is used to separate the nanostructures  14  from the 2D material  12  in some embodiments. In an embodiment, the shaking procedure is performed and combined with the above-mentioned bonding, adhering, or electrostatic attraction method. In an embodiment, the shaking procedure comprises purging a fluid, such as a gas (e.g., air, N 2 , Ar) or a liquid (e.g., water, or solution), to the nanostructures  14 , so that the nanostructures  14  are separated from the 2D material  12 . In an embodiment, the shaking procedure comprises creating a partial vacuum (e.g., by a vacuum pump) with a gas flow to suck the nanostructures  14  and then the nanostructures  14  are separated from the 2D material  12 . In an embodiment, the shaking procedure comprises vibrating and/or pulling the nanostructures  14  by a device, such as a vibrator or a robot. 
     In an embodiment, an additional adhesive layer (e.g. polymethylmethacrylate (PMMA)) may be spin-coated on the nanostructures  14  and the 2D material  12  ( FIG. 81 ) to avoid earlier separation before the transfer. 
     In an embodiment, the flat surface morphology  121  and/or 3D surface morphology  122  of 2D material  12  formed on the first substrate  10  can be repeatedly used for growing another batch of nanostructures  14  after cleaning. This feature can save a lot of material cost, process cost, and time. In an embodiment, a portion, typically less than 50%, of the bottom area of one or more nanostructures  14  has the residue of 2D material coupled to the nanostructures  14  after the separating. The residue can be easily removed by a clean procedure if necessary. 
     In an embodiment, the transferred one or more nanostructures  14  are used to produce one or more layers of devices, e.g., light-emitting diodes, piezoelectric transistors, sensors (e.g. piezoelectric pressure sensors, image sensors, biosensors, or piezo-phototronic effect enhanced sensors), nanogenerators, solar cells, piezo-phototronic effect enhanced solar cells, or chemical reaction cells (e.g. photoelectrochemical water-splitting cells, piezoelectric effect enhanced photoelectrochemical water-splitting cells, or fuel cells). In this text, the term “a light-emitting diode (LED)” refers to a semiconductor device for lighting, displaying, optical sensing, and/or other applications and, and such devices may include, but are not limited to: LEDs, laser diodes (LDs), micro or pixel LEDs, micro or pixel LDs, piezo-phototronic effect enhanced LEDs or LDs, or piezo-phototronic effect enhanced micro or pixel LEDs or LDs. The device can be flexible by transferring the nanostructures  14  to a flexible substrate. In an embodiment, the second substrate is a permanent substrate, i.e., a component of the device. In an embodiment, the transferred one or more nanostructures  14  are used to produce an array of one or more semiconductor devices such as light emitting diodes (LEDs) comprising semiconductor or 2D material layers or p-n diodes or p-i-n diodes or being designed to perform a predetermined electronic function (e.g. flexible diode, transistor, pressure sensor, or integrated circuit) or photonic function (e.g., flexible micro-LED, photodetector, or micro-laser). In an embodiment, each nanostructure may include one or more semiconductor (e.g., silicon) or 2D material layers with controlled dopant concentrations. In this text, the term “array” refers to one or more objects arranged in order or in a particular way. In an embodiment, the p-n diodes or p-i-n diodes may include a compound semiconductor having a bandgap corresponding to a specific region in the spectrum. For example, each p-n diode or p-i-n diode may include one or more layers based on 2D material (e.g. BN, graphene, MoS 2 , WS 2 , or WSe 2 ) or II-VI materials (e.g. ZnSe) or III-V nitride materials (e.g. GaN, AlN, InN), and ternary (e.g. indium gallium arsenide (InGaAs)) and quaternary (e.g. aluminium gallium indium phosphide (AlInGaP)) and other possible alloys of the foregoing materials. In this text, the term “III-V” refers to a substance composed of two or more elements selected from groups III and V, the term “II-VI” refers to a substance composed of two or more elements selected from groups II and VI, and so forth. In an embodiment, an upper and/or outer shell made of 2D material can be further formed on top and/or sidewalls of the nanostructures, and the morphology of 2D material may be single atomic layer or multi atomic layer of disk or shell ( FIG. 8J ). In an embodiment, metallic or semiconductor nanoparticles (NPs) can be coated on the nanostructures (e.g. Platinum (Pt) NPs or graphene oxide quantum dots as catalysts for chemical reaction (e.g. photoelectrochemical water splitting or so called photoelectrochemical hydrogen production). In an embodiment, catalyst nanoparticles, e.g., Rh/Cr 2 O 3  core/shell NPs, may be coated on nanostructures for chemical reaction (e.g. photochemical water splitting), and the coating process can be performed before or after separating nanostructures from the 2D material. In some embodiments, methods (e.g.,  FIGS. 1-3 ) described in this invention are used to generate a three-dimensional integrated circuit (3D IC), which is an integrated circuit by stacking same or different the above-mentioned devices and interconnecting them vertically so that they behave as a single device. In an embodiment, the transferred one or more nanostructures  14  are used to fabricate a first device or a first device array, and one or more steps of  FIG. 1  are repeated to generate or separate more (same or different) nanostructures, which are used to fabricate one or more devices or device arrays stacked on the first device or the first device array. 
       FIG. 3  is a schematic diagram illustrating that one or more nanostructures are used to produce a light-emitting diode or a laser diode in accordance with an embodiment of this invention. Referring to  FIG. 3 , each nanostructure  14  comprises an n-type III-V (e.g., GaN, InGaN, AlGaN)) or II-VI (e.g. ZnSe, CdTe, CdZnSe, or ZnO) nanorod  141 , one or more III-V (e.g., indium gallium nitride (InGaN)) and/or II-VI and/or 2D material (e.g. BN, graphene, or MoS 2 ) nanodisks  142  disposed on the n-type III-V or II-VI nanorods  141 , and a p-type III-V or II-VI nanorod  143  disposed on top of the one or more III-V and/or II-VI and/or 2D material nanodisks  142 . If the number of one or more III-V and/or II-VI and/or 2D material nanodisks  142  is equal to or greater than two, an III-V (e.g., GaN) or II-VI or 2D material barrier  144  may be interposed between each two of the III-V and/or II-VI and/or 2D material nanodisks  142 . In addition, an electrode (not shown), metal/dielectric layer coating (for nanorod lasing), and/or other functional layers (e.g. 2D materials) may be formed on the p-type III-V or II-VI or 2D material nanorods  143  before the transfer. In another embodiment, III-V or II-VI nanorod  141  is p-type, and III-V or II-VI nanorod  143  is n-type. 
       FIG. 4  is a schematic diagram illustrating that one or more nanostructures  14  are used to produce a light-emitting diode in accordance with another embodiment of this invention. Referring to  FIG. 4 , each nanostructure  14  comprises an n-type III-V (e.g., GaN) or II-VI (e.g., ZnO) or 2D material (e.g. MoS 2 ) core  145  surrounded by multiple quantum well (MQW) sheath  146  and a p-type III-V (e.g., GaN) or II-VI (e.g., ZnO) or 2D material (e.g. MoS 2 ) outer shell  147  on the MQW sheath  146 . The multiple quantum well (MQW) sheath  146  consists of two or more III-V (e.g, In x Ga 1-x N) and/or II-VI and/or 2D material (e.g. MoS 2 ) layers  1461  and an III-V (e.g., GaN) or II-VI (e.g., ZnO) or 2D material (e.g. BN) barrier layer  1462  interposed between each two of the III-V (e.g., In x Ga 1-x N) and/or II-VI (e.g., ZnO) and/or 2D material (e.g. MoS 2 ) layers  1461 . The nanostructures  14  can be grown by MOCVD technique. In addition, an electrode (not shown), metal/dielectric layer coating (for nanorod lasing), and/or other functional layers (such as 2D materials) may be formed on the p-type III-V or II-VI nanorods  147  or other portions of nanorods  14  before the transfer. In another embodiment, III-V or II-VI core  145  is p-type, and III-V or II-VI or 2D material outer shell  147  is n-type. 
       FIG. 5  is a schematic diagram illustrating that one or more nanostructures are used to produce a laser diode in accordance with an embodiment of this invention. Referring to  FIG. 5 , each nanostructure  14  comprises a first layer (not shown) of III-V or II-VI nanorod, a lower distributed Bragg reflectors (DBRs) consisting of alternating III-V (e.g., GaN/AlN) and/or II-VI and/or 2D material (e.g. MoS 2 ) ( 148 / 149 ) nanodisks (or nanoshells) disposed on the first layer, an n-type III-V (e.g., gallium nitride) or II-VI nanorod  141  (or nanodisk/nanoshell) disposed on the lower distributed Bragg reflectors ( 148 / 149 ), one or more III-V (e.g., indium gallium nitride (InGaN)) and/or II-VI and/or 2D material (e.g. MoS 2 ) nanodisks  142  disposed on the n-type III-V or II-VI nanorod, a p-type III-V (e.g., GaN) or II-VI (e.g., ZnSe or ZnO) nanorod  143  disposed on top of the one or more III-V and/or II-VI and/or 2D material (e.g. MoS 2 ) nanodisks  142 , and an upper distributed Bragg reflectors (DBRs) consisting of alternating III-V (e.g., GaN/AlN) and/or II-VI (e.g., ZnO) and/or 2D material ( 148 / 149 ) nanodisks disposed on the p-type III-V or II-VI nanorod  143 . If the number of one or more III-V and/or II-VI and/or 2D material (e.g. MoS 2 ) nanodisks  142  is equal to or greater than two, an III-V (e.g., GaN) or II-VI or 2D material (e.g. BN) barrier  144  is interposed between each two of the III-V and/or II-VI and/or 2D material (e.g. MoS 2 ) nanodisks  142 . In addition, an electrode (not shown), metal/dielectric layer coating (for nanorod lasing), and/or other functional layers (such as 2D materials) may be formed on the upper distributed Bragg reflectors (DBRs) or formed on other portions of nanostructures  14  before the transfer. In another embodiment, III-V or II-VI nanorod  141  is p-type, and III-V or II-VI nanorod  143  is n-type. 
       FIG. 6A  is a cross-sectional view illustrating an LED array in accordance with an embodiment of this invention. While some embodiments are described with specific regard to LED array, it is to be appreciated that embodiments of the invention are not so limited and that certain embodiments may also be applicable to other type of components. Referring to  FIG. 6A , nanostructures  14  are formed on a 2D material and separated from the 2D material using the method shown in  FIG. 1 . Nanostructures  14  may comprise p-i-n diodes or p-n diode as shown in  FIG. 3  or  FIG. 4  or  FIG. 5  or comprise one or more semiconductors (e.g., silicon, III-V, II-VI) or single or multi atomic layers of 2D material (e.g. MoS 2 ) layers with controlled dopant concentrations or comprise a configuration designed to perform a predetermined electronic function or photonic function. Driver contacts  18  are formed on a flexible substrate  17 , which may be, but is not limited to, a display substrate or a lighting substrate. A first electrode layer  19  may be formed on each of the driver contact  18 . Optionally a barrier layer (not shown) may be further included in the first electrode layer  19 , which may be made of a high work-function metal such as Ni, Au, Ag, Pd, and Pt or a low work-functional metal such as Al or In, depending on the polarity (n-type or p-type) of contacted portion of the nanostructure  14 . In an embodiment, the first electrode layer  19  may be reflective to light emission. In another embodiment, the first electrode layer  19  may also be transparent to light emission. Transparency may be accomplished by making the electrode layer very thin or using transparent electrodes (such as indium tin oxide) to minimize light absorption. Barrier layer may be made of, but is not limited to, Pd, Pt, Ni, Ta, Ti and TiW. Barrier layer may prevent the diffusion of components into the p-n diode or p-i-n diode. As previously described, a head or a second substrate is used to bond the top surfaces of nanostructures  14  formed on the 2D material, and then the nanostructures  14  are separated from the 2D material. The head or the second substrate may release the separated nanostructures  14  to the flexible substrate  17  with each nanostructure  14  being placed over a driver contact  18 . A dielectric layer  20 , such as silicon nitride or silicon oxide layer, may then be formed to surround each of the nanostructure  14  but expose the top surface of the nanostructure  14 . A second electrode layer  21  may then be formed over the dielectric layer  20  and contact with the top surface of each nanostructure  14 . The second electrode layer  21  may be made of a high work-function metal a low work-functional metal depending on the polarity of contacted portion of the nanostructure  14 . In an embodiment, the second electrode layer  21  may be a common contact line formed over a series of red-emitting (R), green-emitting (G) or blue-emitting (B) micro LEDs or formed over all micro LEDs within a pixel. In an embodiment, the second electrode layer  21  may be reflective to light emission. In another embodiment, the second electrode layer  21  may also be transparent to light emission. 
       FIG. 6B  is a cross-sectional view illustrating an LED array in accordance with an embodiment of this invention. Referring to  FIG. 6B , nanostructures  14  are formed on a 2D material using the method shown in  FIG. 1 . Nanostructures  14  may comprise p-i-n diodes or p-n diode as shown in  FIG. 3  or  FIG. 4  or  FIG. 5  or comprise one or more semiconductor (e.g., silicon, III-V, II-VI) single or multi atomic layers of 2D material (e.g. MoS 2 ) layers with controlled dopant concentrations or comprise a configuration designed to perform a predetermined electronic function or photonic function. Driver contacts  18  are formed on a flexible substrate  17 , which may be, but is not limited to, a display substrate or a lighting substrate. A first electrode layer  19  may be formed on each of the driver contact  18 . The first electrode layer  19  is then used a bonding layer to bond the top surfaces of nanostructures  14  formed on the 2D material, and then the nanostructures  14  are separated from the 2D material. A dielectric layer  20 , such as silicon nitride or silicon oxide layers, may then be formed to surround each of the nanostructure  14  but expose the top surface of the nanostructure  14 . A second electrode layer  21  may then be formed over the dielectric layer  20  and contact with the top surface of each nanostructure  14 . In an embodiment, the second electrode layer  21  may be a common contact line formed over a series of red-emitting (R), green-emitting (G) or blue-emitting (B) micro LEDs. 
       FIG. 6C  is a cross-sectional view illustrating an LED array in accordance with an embodiment of this invention. The LED array of this embodiment has configuration similar to that of  FIG. 6A . The difference between them is that the nanostructures  14  are obliquely formed on the 2D material instead of being vertically aligned. 
       FIG. 6D  is a cross-sectional view illustrating an LED array in accordance with an embodiment of this invention. The LED array of this embodiment has configuration similar to that of  FIG. 6B . The difference between them is that the nanostructures  14  are obliquely formed on the 2D material instead of being vertically aligned. In addition, a sidewall reflector (not shown) may be formed between two individual diodes having different defined pixel or array color to avoid optical cross talk from each other. The sidewall reflector may consist of metal core/dielectric shell. 
       FIG. 6E  is a cross-sectional view illustrating a light distribution of the LED array of  FIGS. 6C and 6D . Referring to  FIG. 6E , lights emitted from the oblique nanostructures  14  can be random with no specific direction, and the surface of the flexible substrate  17  may be coated or deposited a reflector to reflect light emitted from the nanostructures  14 . 
       FIG. 6F  is a cross-sectional view illustrating a light distribution of the LED array of  FIGS. 6C and 6D . Referring to  FIG. 6F , lights emitted from the oblique nanostructures  14  can be random with no specific direction, and are transmitted through the (transparent) flexible substrate  17 . 
       FIG. 6G  is a cross-sectional view illustrating a light distribution of the LED array of  FIGS. 6C and 6D . Referring to  FIG. 6G , lights emitted from the oblique nanostructures  14  can be random with no specific direction, and the second electrode layer  21  or the surface of the second electrode layer  21  reflects the lights through the flexible substrate  17 . 
     Referring to  FIGS. 6E, 6F, and 6G , the LED array with obliquely aligned nanostructures  14  (e.g., p-n or p-i-n diodes) could produce good results or effects. For example, if the LED array is used as a light source, it can emit light with uniform light distribution. Inorganic LEDs or LDs are inherently directional with regards to their distribution and this is amplified when the packaging of these devices includes reflector cups. These devices produce spotlight type distributions that are not always suitable for the final product, and probably results in the end user observing a non-uniform light distribution (bright spots and streaks) caused by the bright on-axis light. Manufacturers avoided these issues by adding heavy diffuser plates to limit bright spotlighting and adding strips of LEDs pointed in multiple directions and locations that require illumination. There have been significant challenges in distributing the light efficiently where desired. In contrast, lights emitted from the nanostructures  14  of this invention can be random with no specific direction, and therefore a uniform light distribution can be achieved. This feature allows manufacturers produce light sources with uniform and efficient lighting distributions and thermal management without using expensive and complicated light guides and diffusers, which requires a large heat sink to avoid the device degradation due to overheat. 
     Referring to  FIGS. 6A-6G , in one embodiment the nanostructures  14  are preferably transferred to a flexible substrate  17  for construction of piezoelectric devices in formats that are thin, flexible and, in some cases, mechanically stretchable. The piezoelectric devices may include, but is not limited to, piezoelectric transistors, piezoelectric pressure sensors, nanogenerators, or piezo-phototronic effect enhanced sensors, solar cells, LEDs, or water-splitting cells. The experimental results show that the produced flexible piezoelectric devices with performance characteristics that can match and even are superior to those of conventional, rigid devices. 
     In a particular embodiment of this invention, an efficient approach is demonstrated to directly transfer obliquely aligned single-crystalline GaN nanorods without process damage using three-dimensional (3D) oriented graphene nanosheets grown on Si (100) substrates. The transfer technique can be easily integrated with the fabrication of a transparent, flexible vertically integrated nanogenerator (VING) with high performance. The nanocrystalline surface of the 3D oriented nanosheets reduces the contact force at the GaN nanorod/graphene interface for the direct transfer, while the 3D oriented surface morphology leads to the oblique nanorod alignment. The flexible VING using the transferred GaN nanorods converted mechanical deformation into electric energy with a high output voltage of up to 8 V and output current of 1.2 μA. This example indicates that nanocrystalline graphene or other sp 2 -bonded two-dimensional (2D) materials could be applicable to grow and transfer single-crystalline III-V and II-VI nanorod arrays,[24] providing a new path to integrate entire layers of nanostructures in arbitrary systems for a wide range of applications. 
     The methods, measurements, and results of the particular embodiment are disclosed as follows. 
     The wafer-scale 3D oriented graphene nanosheets were grown on Si (100) wafers using a remote radio frequency (13.56 MHz) plasma-enhanced chemical vapor deposition (remote PECVD) system. A clean Si substrate was placed in the center of a quartz tube mounted inside the remote PECVD system. The 3D oriented graphene nanosheets were grown without any catalysts or intermediates layer at 400-700° C. with CH 4  (100 mTorr) plasma for 1 hr. The GaN nanorods were grown on the 3D oriented graphene nanosheets without any catalysts by an ultrahigh-vacuum radio-frequency plasma-assisted molecular beam epitaxial (UHV PA-MBE) system under a nitrogen-rich environment with a high substrate temperature fixed at 600-850° C. The obliquely aligned GaN nanorods were obtained by glancing-angle epitaxy with the incident molecular beam subtended an angle of approximately 60° relative to the textured graphene surface at a relatively low growth temperature (600-850° C.). Consequently, Ga adatoms were less mobile, and adsorbed on the sites as they landed, resulting in the oblique alignment. The strain of the nanogenerators can be adjusted by placing different lengths of objects in between the walls to control the wall distances. 
       FIG. 7A  is a schematic diagram showing the growth and transfer method for obliquely aligned GaN nanorods in accordance with an embodiment of this invention. The method consists of the following steps: step (a) growth of the 3D oriented graphene nanosheets  12  on a Si (100) substrate  10 ; step (b) epitaxy of obliquely aligned single-crystalline GaN nanorods  14  on the 3D oriented nanosheets  12 ; and step (c) release of the entire GaN nanorod array from the 3D oriented graphene nanosheets  12  using a tape  15  supported on a polyethylene terephthalate (PET) substrate. In addition, to fabricate a flexible electronic device, the GaN nanorods  14  were transferred onto the transparent flexible ITO-coated PET substrates  16 , as shown in step (d). The tape  15  was composed of three layers, a PET thin film sandwiched between two adhesive PMMA layers with a total thickness of approximately 5 μm, which also acted as an insulating layer for the capacitor-type VING devices in this example.  FIG. 7B  is a picture showing the obliquely aligned GaN nanorods  14  being easily separated from graphene  12  by tape  15  in accordance with an embodiment of this invention. Referring to  FIG. 7B , the GaN nanorods array  14  with a centimeter length scale (4 cm×1.5 cm) can be entirely transferred at a time. According to the method of this invention, the conventional process of spin coating PMMA as a supporting medium onto the nanorods for transfer by wet etching was avoided.[22] The 3D oriented graphene nanosheets with a variety of morphologies, such as petal-, turnstile-, maze-, and cauliflower-like shapes, have been grown, and the morphology is dependent on the type of plasma source and a series of growth parameters, such as the gas type, gas composition, and gas concentrations, chamber pressure, growth temperature, and plasma power.[25]  FIG. 8A  and  FIG. 8B  are SEM images showing the grown of graphene is controlled so that a portion of its surface morphology is flat and the other portion is 3D oriented.  FIG. 8C  is a SEM image showing the petal-like morphology of the 3D oriented graphene nanosheets used in  FIG. 7A .  FIG. 8D  is a high-magnification SEM image showing that the graphene was nanocrystalline with a grain size ˜30 nm, and the nucleation of the 3D oriented graphene nanosheets initiated at the grain boundaries of the nanocrystalline graphene. Next, high-quality obliquely aligned GaN nanorods were epitaxially grown on the 3D oriented graphene nanosheets without catalysts or intermediate layers using PA-MBE. The top-view and cross-sectional SEM images of the GaN nanorods are shown in  FIG. 8E  and  FIG. 8F , respectively, and show approximately 2 μm-long GaN Nanorods with an approximately 60° oblique angle.  FIGS. 8G and 8H  are SEM images showing the 3D oriented graphene nanosheets after the GaN nanorods are separated.  FIG. 8I  is a SEM image showing GaN nanocones grown on the 3D oriented graphene nanosheets.  FIG. 8J  is a SEM image showing lotus-like graphene nanoleafs and graphene nanoshells are further grown on top surfaces and sidewalls of the GaN nanorods that are grown on the 3D oriented graphene nanosheets. 
     The structure of the obliquely c-axis aligned GaN Nanorods grown on the 3D oriented graphene nanosheets was analyzed in detail using transmission electron microscopy (TEM). The low-magnification cross-sectional TEM image of the ˜2-μm-long GaN Nanorods grown on 3D oriented graphene nanosheets is shown in  FIG. 9( a ) . The high-resolution TEM (HR-TEM) image taken at the interface between the graphene and the Si (100) substrate ( FIG. 9( b ) ) shows the structure of the multilayer graphene with a lattice constant (d) of ˜3.49 Å. A thin layer of native oxide (˜2.2 nm) was observed on the Si substrate, indicating that crystalline alignment was not required to grow the 3D oriented graphene nanosheets; thus, the 3D oriented graphene nanosheets can be grown on any substrates of choice that sustain the process temperatures. The grain size and grain shape of the graphene surface could affect the morphology of the GaN nanorods. Indeed, the higher magnification TEM images ( FIG. 9( c )  and  FIG. 9( d ) ) show that the diameter (D) of the GaN Nanorods at the GaN/graphene interface was relatively smaller, approximately 10 nm, than that of the Nanorods grown on transferred graphene with the grain sizes larger than 500 nm (D=˜40 nm by PA-MBE [21] and ˜150 nm by metal-organic vapor phase epitaxy (MOVPE) [20]). The radial growth was prominent along the axial direction ( FIG. 9( c ) ), and the diameter reached a maximal value of ˜150 nm at approximately 1 μm away from the bottom. As the nanorods were grown longer than 1 μm in length, they reached a self-equilibrated state; thus, the diameter remained constant, approximately 50 nm at the top. In contrast to the double truncated conical GaN nanorods grown on the 3D oriented graphene nanosheets, the diameters of nanorods are uniform along the entire nanorods grown on the highly crystalline transferred graphene (D=40 nm and the length was 500 nm by PA-MBE [21], and D=150 nm and the length was 2 μm by MOVPE [20]). Thus, the much smaller diameter at the bottom of the GaN nanorods was influenced by the smaller grain size of the 3D oriented graphene nanosheets for self-organized epitaxial growth. The surface area ratio between the top (D=˜50 nm) and bottom (D=˜10 nm) diameters of the GaN nanorods grown on 3D graphene nanosheets was 25-fold higher than that of the nanorods grown on the highly crystalline transferred graphene; thus, the separation of the entire array of GaN nanorods can be easily achieved by using only handling tape. The selected-area electron diffraction (SAED) patterns shown in the inset of  FIG. 9( e )  confirmed that the GaN nanorods were single crystals with growth along the c-axis direction. The lattice constant (c) (˜5.185 Å) and c/a ratio (˜1.626) of the GaN nanorods ( FIG. 9( e ) ) were identical to the intrinsic lattice constants of wurtzite GaN, [26] indicating that the GaN nanorods grown on the nanocrystalline graphene surface were nearly strain-free single crystals. 
     The crystal quality and optical properties of the grown GaN nanorods were characterized using room temperature photoluminescence (RT-PL) using a spectrometer with a 325 nm excitation light source from a He—Cd laser. The PL spectra in  FIG. 10( a )  are almost identical for the GaN nanorods grown on the 3D oriented graphene nanosheets (blue curve) and on the single-crystalline Si (111) substrate (black curve); the cross sectional SEM image is shown in the inset of  FIG. 10( a ) . Similar to the PL for the vertical GaN nanorods grown on Si (111), the PL spectrum for the oblique GaN nanorods grown on the 3D oriented graphene nanosheets exhibited a strong near-band-edge (NBE) emission at 363 nm (3.41 eV) with a full width at half maximum (FWHM) of 55 meV ( FIG. 10( b ) ). In addition to the 3.41 eV NBE emission peak, two phonon replicas (peaks at 3.35 eV and 3.25 eV) are also clearly visible in  FIG. 10( b ) . [27,28] Defect-related emission was not observed for the GaN Nanorods, such as the broadband yellow emission (with peaks at 550˜560 nm) that is frequently exhibited by GaN nanorods that are grown using various techniques.[20,28-30] The absence of defect emissions in the PL spectra and the single-crystalline structure from the TEM characterizations indicate that high-quality GaN nanorods were achieved by using the low-crystalline graphene substrates. 
     To demonstrate the functionality of the directly transferred obliquely aligned GaN nanorods under deformation, this invention constructed a transparent flexible capacitor-type flexible vertically integrated nanogenerator (VING) with an active area of ˜6 cm 2  containing the GaN nanorods/5 μm-thick tape sandwiched between two ITO electrodes deposited on a PET substrate, as shown in step (d) of  FIG. 7A .  FIG. 11A  shows three distinctively different states corresponding to the original flat state, bending state, and straightening state of the nanogenerator mounted on the bending stage to generate output voltage and current. The two ends of the nanogenerator were fixed on the two walls of a linear motion stage with a spring connecting the walls.  FIG. 11B  is a chart illustrating the correlation between a magnified output voltage and the bending conditions of the nanogenerator shown in  FIG. 11A .  FIG. 11C  is a chart illustrating an output voltage and current of the nanogenerator shown in  FIG. 11A . In B, the open-circuit voltage under the bending state is nearly constant (dashed green line), because the piezoelectric polarization charges occurred on the top and bottom surfaces of the GaN Nanorods. Asymmetric sharp peaks from the output voltage were observed upon bending (green circle) and straightening (red circle) the device, and the negative voltage and current peaks became larger than the positive peaks as the strain increased from 0.16% to 0.58%, as shown in  FIG. 11B  and  FIG. 11C , respectively. In the linear motion system, the bending rate became slower and straightening rate became faster as the strain increased. The asymmetric AC power peaks were attributed to the larger (smaller) shaking from the GaN nanorods in response to the faster straightening (slower bending) as the strain increased, indicating that the output peak intensity correlated well to the force profile. The maximal peak voltage and current were 8 V and 1.2 μA (0.2 μA/cm 2 ), respectively, under the straightening force induced by the 0.58% strain. These values were ˜100-fold larger than those of the vertically oriented GaN nanorods VINGs assembled on bulk Si (111) substrates under a compressive force (0.08 V and 0.01 μA for 1 cm 2  active area).[32,33] The maximum output voltage and current in this embodiment were more than 6 times larger than the former best GaN nanorod VINGs under a bending force (output voltage 1.2 V, output current 40 nA), in which the nanorods were laterally aligned on flexible substrates with an uncontrolled nanorod orientation.[34] Moreover, previous experimental and simulation results revealed that obliquely bended nanorods exhibited a piezoelectric potential that was more than 2 times larger than that of lateral bended and compressed nanorods grown along the c-axis.[33,35,36] The high performance of the GaN nanorod based VING of this embodiment was partially attributed to the oblique alignment, which efficiently caused the nanorods to be bended obliquely under the bending/straightening forces. Piezoelectric VINGs using vertically aligned ZnO nanorods have been well studied, and the theoretical calculations have indicated that the piezoelectric potentials of the ZnO nanorods are ˜2.5 times larger than that for GaN nanorods based on their piezoelectric constants, Poisson ratios, Young&#39;s moduli, and relative dielectric constants.[34,35] The former best flexible ZnO Nanorods VING output a voltage of 5 V and current of 0.3 μA/cm 2  under a 0.12% strain using post-annealed, double-layered ZnO nanorods with a 150-nm in diameter and 2-μm length.[37] This embodiment shows results that are qualitatively in good agreement with the theoretical and experimental results [34,35,37], indicating that the epitaxy, transfer, and VING integration approaches are reliable for flexible piezoelectrics. 
     In summary, the embodiment of this invention is the first demonstration of simple transfer of wafer-scale single-crystalline GaN Nanorods grown on 3D oriented graphene nanosheets using handling tape without requiring a wet-etching process. The direct transfer resulted from the self-organized epitaxy of GaN nanorods on nanocrystalline 3D oriented graphene nanosheets; this method enables reducing the contact area at the interface between the GaN nanorods and 3D oriented graphene nanosheets. The oblique alignment of the GaN nanorods obtained from the textured 3D oriented graphene surface is important for inducing higher piezopotentials along nanorods during oblique bending to provide a practical functionality for piezoelectric energy harvesters and sensors. A high performance transparent, flexible, obliquely aligned nanorod-embedded piezoelectric VING was successfully fabricated. The flexible VING converted mechanical deformation into electric energy using the transferred GaN nanorods with high output voltage up to 8 V and an output current of 1.2 μA (0.2 μA/cm 2 ). Additional levels of performance optimization of the transferred nanorods embedded flexible VING could be achieved by passivating the surfaces of the nanorods [37] and segmenting the VING using lithography [38] to prevent the current leakage through the internal structure of the nanorods. The enhanced piezoelectricity that is offered by these obliquely aligned GaN nanorods integrated on flexible substrates could offer immediate and substantial practical implications for emerging applications involving with the piezoelectronic, piezotronic and piezo-phototronic effects.[6-9,11,12] 
     Conditional language, such as, among others, “can,” “could,” “might,” or “may,” unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that embodiments include, and in other interpretations do not include, certain features, elements and/or steps. Thus, such conditional language is not generally intended to imply that features, elements and/or steps are in any way required for one or more embodiments, or interpretations thereof, or that one or more embodiments necessarily include logic for deciding, with or without user input or prompting, whether these features, elements and/or steps are included or are to be performed in any particular embodiment. 
     Although specific embodiments have been illustrated and described, it will be appreciated by those skilled in the art that various modifications may be made without departing from the scope of the present invention, which is intended to be limited solely by the appended claims.