Facile growth of high aspect ratio c-axis GaN nanowires and their application as flexible p-n NiO/GaN piezoelectric nanogenerators

Johar, Muhammad Ali,Waseem, Aadil,Hassan, Mostafa Afifi,Kang, Jin-Ho,Ha, Jun-Seok,Lee, June Key,Ryu, Sang-Wan Elsevier 2018 Acta materialia Vol.161 No.-

<P><B>Abstract</B></P> <P>Piezoelectric nanogenerators (PNGs) have attracted great interest as energy sources to power-up smart clothing, micro/nano systems, and portable electronic gadgets. Due to non-centrosymmetric crystal structure, bio-compatibility, and mechanical robustness of GaN, it is a promising candidate to fabricate PNGs. In this study, c-axis GaN nanowires were grown by MOCVD, then were embedded inside polydimethylsiloxane and flipped on to the flexible substrate, followed by the deposition of p-type NiO to form heterojunction. The fabrication of GaN nanowires based heterojunction PNG on flexible substrate is the first report to the best of our knowledge. The piezoelectric properties of PNGs were investigated as a function of the GaN nanowire length. A maximum piezoelectric output potential of 20.8 V and current of 253 nA were measured. The stability of the device was also evaluated and found stable even after 20,000 cycles. This high piezoelectric output was attributed to the suppression of free carrier screening and junction screening. Moreover, the underlying reasons for the high stability are the malleability of the device and high aspect ratio of the GaN nanowires. The design and stability of our device make it a promising candidate for applications in self-powered systems for environment monitoring and low power electronics.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Controlled conductivity of p-type Cu_xO/GaN piezoelectric generator to harvest very high piezoelectric potential

Johar, Muhammad Ali,Kang, Jin-Ho,Ha, Jun-Seok,Lee, June Key,Ryu, Sang-Wan ELSEVIER SCIENCE 2017 JOURNAL OF ALLOYS AND COMPOUNDS Vol.726 No.-

<P><B>Abstract</B></P> <P>Typically, semiconductor piezoelectric-generators (PGs) exhibit low output potential because of the rapid screening by mobile carriers, and the output potential drops within fractions of a second. The reduction of internal carrier screening has been attempted by several groups, but complete suppression has not yet been reported. In this paper, a novel method for the fabrication of high-power PGs is presented. The detrimental screening issue was solved by fabricating a p–n heterojunction of p-type Cu<SUB>x</SUB>O (x = 1, 2) and GaN. This was found to be an effective structure for decreasing the internal screening, while the suppression of screening was directly proportional to the resistivity of the p-type layer. The resistivity of the p-type thin film was controlled by optimizing the oxygen content in the environment during deposition. From X-ray diffraction analysis, the crystal structure of copper oxide changed by varying the oxygen incorporation, which primarily controlled the p-type conductivity. The optimized device exhibited output voltage of 26 V with a current density of 11.40 μA cm<SUP>−2</SUP> under a pressure of 5 MPa. This study provides an effective approach for generating high electrical power, and it would be advantageous to construct flexible GaN PGs, which are essential devices for self-powered sensor networks.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Controlled conductivity of p-type Cu<SUB>x</SUB>O for Cu<SUB>x</SUB>O/GaN PG was fabricated. </LI> <LI> Junction screening was addressed to enhance piezoelectric output. </LI> <LI> Piezoelectric output of PG was 26 V and 11.4 μA cm<SUP>−2</SUP> with an efficiency of 2.6%. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

A scalable, flexible and transparent GaN based heterojunction piezoelectric nanogenerator for bending, air-flow and vibration energy harvesting

Johar, Muhammad Ali,Kang, Jin-Ho,Hassan, Mostafa Afifi,Ryu, Sang-Wan Elsevier 2018 APPLIED ENERGY Vol.222 No.-

<P><B>Abstract</B></P> <P>Flexible functional devices are extremely suitable for malleable, sustainable, and portable applications such as smart clothing, flexible electronics and medical applications. Here, we present a scalable, transparent, and flexible piezoelectric nanogenerator (STF PNG) fabricated by forming a p-n NiO/GaN heterojunction using an electrochemical lift-off process to transfer GaN onto a flexible substrate. Several actuation sources such as air-flow, finger forces for bending, vibrations with a frequency of 20 Hz, and cyclic stretching-releasing agitation by a linear motor were applied to generate piezoelectric bias. Peak piezoelectric output voltage and current of 30 V and 1.43 µA, respectively, were measured. Such high piezoelectric bias was generated by suppressing free carrier screening and junction screening; the former was achieved due to removal of compressive stresses from GaN after the lift-off process, while the latter was achieved by the deposition of a highly resistive p-type NiO layer on transferred GaN and a sandwiched Polydimethylsiloxane resulting in a very high junction resistivity of the p-n NiO/GaN heterojunction STF PNG. As a result, our approach provides a new strategy for novel and highly efficient design of semiconductor-based flexible PNGs for a wide variety of applications.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A novel fabrication of GaN based heterojunction flexible piezoelectric generator. </LI> <LI> Piezoelectricity from vibrations, finger force, air-flow, and cyclic agitation. </LI> <LI> High electrical energy generation from ambient vibrations. </LI> <LI> High output voltages by suppressing internal screening and junction screening. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Controlled carrier screening in p-n NiO/GaN piezoelectric generators by an Al₂O₃ insertion layer

Johar, Muhammad Ali,Jeong, Dae Kyung,Hassan, Mostafa Afifi,Kang, Jin-Ho,Ha, Jun-Seok,Lee, June Key,Ryu, Sang-Wan IOP 2017 Journal of Physics. D, Applied Physics Vol.50 No.48

<P>The performance of a piezoelectric generator (PG) depends significantly on the internal screening process inside the device. As piezoelectric charges appear on both ends of the piezoelectric crystal, internal screening starts to decrease the piezoelectric bias. Therefore, the piezoelectric energy generated by external stress is not fully utilized by external circuit, which is the most challenging aspect of high-efficiency PGs. In this work, the internal screening effect of a NiO/GaN p-n PG was analyzed and controlled with an Al<SUB>2</SUB>O<SUB>3</SUB> insertion layer. Internal screening in the p-n diode PG was categorized into free-carrier screening in neutral regions and junction screening due to charge drift across the junction. It was observed that junction screening could be significantly suppressed by inserting an Al<SUB>2</SUB>O<SUB>3</SUB> layer and that effect was dominant in a leaky diode PG. With this implementation, the piezoelectric bias of the NiO/GaN PG was improved by a factor of ~100 for high-leakage diodes and a factor of ~1.6 for low-leakage diodes. Consequently, NiO/Al<SUB>2</SUB>O<SUB>3</SUB>/GaN PGs under a stress of 5 MPa provided a piezoelectric bias of 12.1 V and a current density of 2.25 <I>µ</I>A cm<SUP>−2</SUP>. The incorporation of a highly resistive Al<SUB>2</SUB>O<SUB>3</SUB> layer between p-NiO and n-GaN layers in NiO/GaN heterojunctions provides an efficient means of improving the piezoelectric performance by controlling the internal screening of the piezoelectric field.</P>

Effect of crystal orientation of GaN/V₂O₅ core-shell nanowires on piezoelectric nanogenerators

Waseem, Aadil,Johar, Muhammad Ali,Hassan, Mostafa Afifi,Bagal, Indrajit V.,Ha, Jun-Seok,Lee, June Key,Ryu, Sang-Wan Elsevier 2019 Nano energy Vol.60 No.-

<P><B>Abstract</B></P> <P>GaN/V<SUB>2</SUB>O<SUB>5</SUB> core-shell nanowire (NW) piezoelectric generators of controlled crystallographic orientations were fabricated, and the device performance was characterized. Catalyst-assisted c- and m-axis GaN NWs were grown on a c-plane GaN thin film by varying the NH<SUB>3</SUB> flow rate and reactor pressure. The grown NWs were then utilized to fabricate the flexible piezoelectric nanogenerators (PNGs) to practically investigate the impact of the c- and m-axis GaN NWs on the piezoelectric response. The c-axis GaN NWs exhibited a higher piezoelectric output than m-axis GaN NWs. Furthermore, the GaN/V<SUB>2</SUB>O<SUB>5</SUB> core-shell structure was utilized in the NWs to suppress the internal carrier screening that degrades the piezoelectric output. The maximum output voltage (27 V) exhibited by core-shell c-axis NWs was thrice the voltage exhibited by pristine c-axis GaN NWs (9 V). A stability test was performed for one hour to verify the feasibility of using flexible PNGs for real applications. The high stability of PNGs was attributed to the flexibility and high crystallinity of the NWs.</P> <P><B>Highlights</B></P> <P> <UL> <LI> C- & m-axis GaN NWs are grown by MOCVD for piezoelectric generators. </LI> <LI> The piezoelectric output for pristine c-axis NWs is twice the output of m-axis NWs. </LI> <LI> The piezoelectric output of the GaN/V<SUB>2</SUB>O<SUB>5</SUB> core-shell NWs is three times of pristine NWs. </LI> <LI> PNGs exhibited long-term stability due to excellent mechanical properties of NWs. </LI> <LI> GaN/V<SUB>2</SUB>O<SUB>5</SUB> heterojunction suppressed the junction current screening effect. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>GaN/V<SUB>2</SUB>O<SUB>5</SUB> core-shell nanowire (NW) piezoelectric generators of controlled crystallographic orientations are reported. Catalyst-assisted c- and m-axis GaN NWs grown by MOCVD are utilized to fabricate the flexible piezoelectric nanogenerators to investigate the impact of the c- and m-axis GaN NWs on the piezoelectric response. The c-axis GaN NWs exhibited the higher piezoelectric output than m-axis GaN NWs. The GaN/V<SUB>2</SUB>O<SUB>5</SUB> core-shell structure is utilized in the NWs which further enhanced the output voltage to 27 V and output current to 590 nA.</P> <P>[DISPLAY OMISSION]</P>

Facile morphology control of high aspect ratio patterned Si nanowires by metal-assisted chemical etching

Bagal, Indrajit V.,Johar, Muhammad Ali,Hassan, Mostafa Afifi,Waseem, Aadil,Ryu, Sang-Wan Springer-Verlag 2018 Journal of materials science Materials in electron Vol.29 No.21

Anchoring MWCNTs to 3D honeycomb ZnO/GaN heterostructures to enhancing photoelectrochemical water oxidation

Patil, Santosh S.,Johar, Muhammad Ali,Hassan, Mostafa Afifi,Patil, Deepak R.,Ryu, Sang-Wan Elsevier 2018 Applied Catalysis B Vol.237 No.-

<P><B>Abstract</B></P> <P>Gallium nitride (GaN) is one of the ubiquitously known photoanode for photoelectrochemical water splitting (PEC-WS) due to its tunable band gap and favorable band edge positions. However, the unavoidable surface defects in GaN induces surface Fermi level pinning and surface band bending which severely reduces its PEC conversion efficiency. Constructing heterostructure is the key to approaching better charge separation efficiency and light harvesting ability for PEC-WS. Considering this, we have fabricated ternary heterostructure of GaN/ZnO/MWCNTs photoanode by combining metal organic chemical vapour deposition (MOCVD), hydrothermal and ‘dip and dry’ methods. FE-SEM results showed the formation of 3D hierarchical honeycomb structure of ZnO on GaN thin film surface when MWCNTs are added into hydrothermal reaction. We investigate the advantage of ZnO honeycomb structure in enhancing the solar PEC-WS performance of GaN photoanode. The synergy of incorporating MWCNTs has resulted into improvement in surface morphology, electron transportation and light harvesting capability. The as obtained ternary heterostructure exhibits photocurrent (<I>J</I> <SUB>ph</SUB>) of 3.02 mA/cm<SUP>2</SUP> at 0 V versus Pt electrode under 1-sun light illumination which is about 2.58 times higher than that of pristine GaN photoanodes (<I>J</I> <SUB>ph</SUB> = 1.14 mA/cm<SUP>2</SUP>).</P> <P><B>Highlights</B></P> <P> <UL> <LI> Constructing GaN/ZnO heterostructure enhances charge separation and PEC performance. </LI> <LI> MWCNTs act as structure directing agent and photosensitizer. </LI> <LI> MWCNTs modified porous hierarchical 3D honeycomb ZnO/GaN heterostructures are presented. </LI> <LI> The ternary heterostructure GaN/ZnO/MWCNTs promises unassisted water splitting. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>Synthesis of ternary heterostructure GaN/ZnO/MWCNTs with porous 3D hierarchical honeycomb structures has been demonstrated. As fabricated ternary heterostructure GaN/ZnO/MWCNTs photoanode promises unassisted PEC water splitting with enhanced photocurrent density of 3.02 mA/cm<SUP>2</SUP>, approximately ∼2.58 times higher than that of pristine GaN photoanode.</P> <P>[DISPLAY OMISSION]</P>

Type-II ZnO/ZnS core-shell nanowires: Earth-abundant photoanode for solar-driven photoelectrochemical water splitting

Hassan, Mostafa Afifi,Johar, Muhammad Ali,Waseem, Aadil,Bagal, Indrajit V.,Ha, Jun-Seok,Ryu, Sang-Wan The Optical Society 2019 Optics express Vol.27 No.4

High-performance ZnS/GaN heterostructure photoanode for photoelectrochemical water splitting applications

Hassan, Mostafa Afifi,Kang, Jin-Ho,Johar, Muhammad Ali,Ha, Jun-Seok,Ryu, Sang-Wan Elsevier 2018 Acta materialia Vol.146 No.-

<P><B>Abstract</B></P> <P>We present a study of ZnS/GaN heterostructure photoanodes fabricated by using ZnS thin films deposited on GaN by atomic layer deposition and a significantly enhanced photoelectrochemical (PEC) water splitting performance was demonstrated. The PEC performance of the photoanodes was investigated for various ZnS thicknesses and GaN doping concentrations. The photocurrent density of the ZnS/GaN photoanode at zero bias was enhanced by a factor of 1.75 compared to that for the reference GaN structure. Furthermore, significantly enhanced photoanode stability was observed with the optimized ZnS coating. The high performance of the ZnS/GaN photoanode is attributed to the type-II band alignment of the heterojunction, which forms a potential barrier for electron injection to the electrolyte while facilitating hole transfer. Therefore, using a ZnS cocatalyst coated on GaN is a promising technique to fabricate photoanodes for PEC-based solar energy harvesting.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Enhanced piezoelectric output of NiO/nanoporous GaN by suppression of internal carrier screening

Waseem, Aadil,Jeong, Dae Kyung,Johar, Muhammad Ali,Kang, Jin-Ho,Ha, Jun-Seok,Lee, June Key,Ryu, Sang-Wan Institute of Physics 2018 Semiconductor science and technology Vol.33 No.6

<P>The efficiency of piezoelectric nanogenerators (PNGs) significantly depends on the free carrier concentration of semiconductors. In the presence of a mechanical stress, piezoelectric charges are generated at both ends of the PNG, which are rapidly screened by the free carriers. The screening effect rapidly decreases the piezoelectric output within fractions of a second. In this study, the piezoelectric outputs of bulk- and nanoporous GaN-based heterojunction PNGs are compared. GaN thin films were epitaxially grown on sapphire substrates using metal organic chemical vapor deposition. Nanoporous GaN was fabricated using electrochemical etching, depleted of free carriers owing to the surface Fermi-level pinning. A highly resistive NiO thin film was deposited on bulk- and nanoporous GaN using radio frequency magnetron sputter. The NiO/nanoporous GaN PNG (NPNG) under a periodic compressive stress of 4 MPa exhibited an output voltage and current of 0.32 V and 1.48 <I>μ</I>A cm<SUP>−2</SUP>, respectively. The output voltage and current of the NiO/thin film-GaN PNG (TPNG) were three and five times smaller than those of the NPNG, respectively. Therefore, the high-resistivity of NiO and nanoporous GaN depleted by the Fermi-level pinning are advantageous and provide a better piezoelectric performance of the NPNG, compared with that of the TPNG.</P>